The reason could be that wings add an element of fantasy and make the manticores stand out. It also allows for more exciting action scenes and gives them the ability to move quickly and freely.
Well, it's often to give them an added advantage in combat or to make them more intimidating. Wings make them seem more powerful and magical.
I think it's because wings symbolize a certain level of supernatural or mythical power. In fantasy fiction, having wings on a manticore makes it more fantastical and helps create a more awe-inspiring image.