Aside from being patronizing and demeaning about whatever happens to Leonor's "cute head" or not, her grandfather had to stop a coup. And he attributes that largely to having known a lot of the participants from the military academy.
She's going to be CINC, not a mascot. She's doing three years. It's not just a show.
Her grandfather stopped a coup, but those were different times when Spain was a relatively new democracy after decades of a fascist dictatorship. Nowadays, Spain is a mature European parliamentary democracy and the possibility of the armed forces ever attempting a coup is close to zero. In fact, even back then, the attempted coup was more like an isolated act of a few extremists and never backed by the professional heads of the armed forces as far as I understand.
As for the CINC role, despite JC's actions during the aforementioned coup, I understand it is mostly ceremonial nowadays, not unlike the role of the British monarch or the Governors General in the realms. The CINC awards military honors and I suppose probably presents colors to military units and approves new military insignia and badges. All officers and enlisted ranks also swear an oath that includes a pledge of loyalty to the King. However, the Spanish constitution says that the responsibility for the military administration and for national defense lies with the government, which is constitutionally separate from the King in Spain.
Realistically speaking, Leonor does not need military training to exercise the role of CINC in the narrow sense that this role is defined, either explicitly or in practice, in modern Spain. Under normal circumstances, she won't have political control over the miitary (that lies with the prime minister and the defense minister) and much less operational command over the forces (which lies with the military chiefs of staff and the unit or field commanders below them). Having said that, I don't think she is "a mascot" either and I believe the training she is getting is for real and that she will get out of the 3 service academies with real military skills. I just don't see a point of doing it unless she really likes or is interested in military life.
King Felipe VI mentioned once that the military education of the Princess was also important because of the values that military life can instill on people and its importance in character building (or something along those lines). I don't disagree with the King that military education indeed has the merits he mentioned and builds character and other commendable virtues, but I disagree that Leonor (and most other people) cannot acquire similar virtues in a civilian life only without having any military experience.
Again my problem is with why it is accepted without any debate that Leonor must be in the military because she is the heir to the Crown as if that were an absolute truth.
Mutatis mutandis, I believe the same question applies to most other heirs (in some countries like the Netherlands, military service doesn't seem to be such a big deal, at least for female heirs, although men might be possibly treated differently).
Both attended colleges based on Kurt Hahn philosophy, only different countries.
I guess it will be a bit difficult for Leonor, as a woman, and she won't do the degree as her classmates will do.
Then going to Uni, older than her classmates.
But she is not an ordinary girl.
From a certain point of view, it could be better to go to uni first and then enter the military academy later as Prince William did.
However, the UK has a completely different model of military education since its military academies are not degree-granting institutions as in the United States, Canada, Belgium, and, I believe, Spain. Instead, the British military academies are more like technical colleges that offer a short officer's commissioning course (44 weeks for example at Sandhurst) after which the junior commissioned officers move on to other additional specialist training courses in their chosen branches (aviation, infantry, signals, intelligence, logistics, etc.). Technically you can join the military academies straight out of secondary school (A-Levels in England), but nowadays in practice, over 80 % of the officer cadets in the British Army, and higher than that in the RAF and possibly in the Royal Navy, finish a 3-year bachelor's degree or a 4-year undergraduate master's degree first before joining a military academy (as William did). Of course, for some specialized branches in the military like for engineers, lawyers, medical doctors, nurses, and chaplains, a previous university degree is already mandatory to join that branch specifically after commissioning.
In comparison, in countries where the military academies are degree-granting institutions, which I think is an increasingly common model in many countries, one would typically leave High School and stay 3 or 4 years in the military academy, earning both a bachelor's degree and an officer's commission in the process. After that, they go directly into military service and do not get a second bachelor's degree from a civilian university although they may earn a master's (or even a PhD) degree later from a civilian university as they progress in their military career. The latter by the way is also common in the UK, where many candidates for promotion to senior officer (especially general officer) later in life usually get another specialized one-year master's degree in defense studies or something similar.
I have a split opinion about which model of military education is better, although I am slightly biased in favor of the US model, but, in any case, Leonor and Elisabeth are exceptions in the sense of getting (partial) military training in the academies without graduating with a degree and then completing a bachelor's degree at a civilian university when they are older. For Leonor, the (full-time) military training phase is just longer than in Elisabeth's case, meaning that she will be even older when she goes to university.
PS: In the US, rather than being a full-time officer cadet in a 4-year degree-granting federal military academy, there is also the flexibility of attending a civilian university and receiving part-time military training in parallel through an ROTC program, but still commissioning as an active-duty officer in the regular armed forces. In fact, surprisingly, most regular (not reserve) junior officers in the US now come from ROTC, rather than the more exclusive and elitist federal military academies.