This question already has an answer here:
Follow up to Why do we don the tallit gadol?:
Two reasons are given in the answers. One is that when praying, we want to appear presentable before the king. Another reason suggests that it better fits the requirement of wearing a garment that covers the body.
If we follow the 1st reason that it is important for prayer, then why doesn't everybody wear it at all prayers? It seems to be worn only during Shacharit. (I'm not talking about the Shat"z as there are different customs for that.)
If it's for the 2nd reason, then why not wear the tallit gadol all day long?
And for both reasons, young kids don't wear these. Many customs don't have people wearing the tallit gadol until one is married. If it's important for prayer, why not have everyone wear it?