"I want to eat" and "I want to be able to eat" don't necessarily mean the same thing. With the second sentence, you may be expressing that presently you are not ABLE to eat. eg you have just come out of surgery. You could say "I want to eat". But if you say "I want to be able to eat", that would carry a different meaning, given that it's clear in that situation that you are presently not allowed to eat.
Mmmm. I would say that this is a far better sentence: "Je veux être capable de manger". Let's say that I do not like violence against animals and that I am planing to become a vegetarian. In the meanwhile every time that I am at the table to have a meal I look at my plate and say: "Je veux être capable de regarder mon assiette et me sentir bien de ce que je mange." (I want to be able to look at my plate and feel good as to what I eat).