Why? Because that is what royals do. That is their job. Their role is purely ceremonial. It is to look good, wave at people, make pretty speeches written by someone else, say the right thing and that is it. Charities are seen as a great avenue for them to use to do these things in aid of but that is all.
Quite, right for the most part however I do not think your description above applies entirely to Catherine...
For the last century and more the British royal family have been ceremonial and not much more.
The powers of Queen Victoria were once described as 'the power to advise, to warn and to be consulted'. Full stop, end of powers.
Well, the Queen, as head of state, has many powers, more than most people think...she just "shares them"
...but that is a topic for another thread
...do something to inspire and encourage the people - that is what the duties are about - helping other people, raising money for charity and raising the profile of various issues.
The provide entertainment through their duties.
IMO for Catherine to take on that approach too soon, in regards to "duties and roles" is a recipe for disaster. She should be given as much time as necessary to make as smooth a transition into her new status as royalty as possible. I don't believe Catherine becoming a patron of a charity and giving speeches to raise funds and awareness with much ado would be a proper method of this approach.
There are going to be an assortment of changes that Catherine will have to confront immediately following her wedding...not just the obvious ones, name (which I know is a topic on another thread)...residence...media perception...but there are also more subtle changes she will face:
i.e. soldiers and sailors of all ranks saluting you...being introduced every where you go...having people twice your age bow and curtsy to you...everyone in a room stand when you enter...the presence of armed personnel (well she may be used to that already)...quite intimidating, wouldn't you agree?
Attendance at royal engagements or ceremonies might, just might, help ease these lifestyle modifications that will initiate as result of Catherine becoming royal. Again IMO it is paramount that Catherine has an unproblematic as possible switch from life as a commoner to life as a royal.