I try to make it short, I have some problems with that
Excuse me, if I chose the wrong words, I do not want to offend anybody.
I absolutely agree: It'll remain a selfish expensive adventure trip to an exotic country, if you don't do anything for these people after you have seen their lifes.
Imho Mary had a genuine interest in visiting these countries. I really think that she got interested in that things due to meetings with the DRC and also due to conversations with other Crownprincesses, I think they also talk about their patronages and travels when they meet. Another point is that people having worked in such countries often become a special community, this makes it often difficult with other people. As a patron of the DRC it is easier for her to be part of that community, she'll better understand the people she is working with.
Uganda was well chosen: Safe enough for her and many, many different NGOs there. She could see what the different problems are (e.g. to name some of them: food variety, high school fees and education = for Ugandans, they also say that education is the key, politicy (!), a lot of orphans, HIV, completely different languages in one country, streets, dependence on support, but also mental problems after these traumatic events, co-dependence on other countries and their political situations in africa...) and she could see how different NGOs cooperate.
This all can help her to become a better patron, also for her own Mary Foundation. People in Uganda need a more stable political situation, more real influence on policy and of course investment so that Ugandan people can earn their own money and become more independent. And now she also knows these quite complex political issues and can call attention to that (and this has not necessarily to be loud and with a lot of press attention).
Ugandans have so much hope: And they are right, but there is still a lot to do. Uganda can be on of the top agriculture countries, the streets are very slowly getting better, IDP-camp inhabitants can go back to their fields (if the mines are removed) and work as farmers and for tourists there are different beautiful national parks in Uganda. Already Winston Churchill has called Uganda "the pearl of africa".