all things like that; public debates, speeches, presentations; are to help students be able to speak in public.
we've all heard of that famous poll whos results included that americans fear public speaking more than death. and it's true, most people (age regardless) hate public speaking.
students are "forced" to do so in school so that they can, in theory, become more comfortable at public speaking and more practiced/experienced, so it is at least easier for them to do later in life if they have already done it so many times (even if, even as adults, they are still not tottaly comfortable)
in almost every job you may want to have, there is at least some amount of public speaking involved (wheter you realize it or not!) and besides that, there are many other cercumstances in which you may find yourself needing to speak publiclly (speaking at a friend's wedding, a rewards ceremony, etc)
besides, you will do it a LOT in school as time goes on, so you may as well get used to it know

i don't like public speaking either (in fact, it's my most hated part of high school), but everyone must learn how to do it, as it will prove to be an important life skill later on.