Why do people use animation software?

Why do people use outside animation software when animating for their games? Why not do everything in persona? I guess what I am trying to ask is what are the restrictions and benefits of animating in persona and what are the restrictions and benefits of using a third party software? Thank you

The basics concept of animation and timeline is universal. People use animation software because it gives them more control when first creating the character , UV mapping, rigging it according to UE4 skeletal mesh requirements, weight painting, texture painting, attachments and still have the ability to tweak the mesh if needed. Secondly most animators are from a background where they have been using an animation software, this makes their skills universal. They can get into Games and Movies, Commercials etc.

There is a talk on exactly what you are asking : UE4 Animation and Physics Technical Showcase | GDC 2017 | Unreal Engine - YouTube