What is the halachik difference between a talit gadol and a talit katan? If the talit gadol is somehow better, why don't we only wear that? If they are equal, why do we need a talit gadol at all?
The advantages of a Tallit Katan is that you get to wear it all the time. The advantage of a tallit gadol is that it is for sure the correct size to be obligated in the mitzva. So, since for many it is impractical to wear a big enough tallit to be for sure obligated all the time, they are at least encouraged to wear a smaller one which might be obligated at all times as this is preferable to not wearing any tzitzit all day. But at least once a day during davening they put on a bigger tallit to for sure fulfill the mitzva once every day. (Based on Mishna Berurah 16:1 quoting Shut Rama 110)