Box collider vs Sprite collider...?

What is the benefit of using a separate object for collisions instead of the animated character?

I will ensure that your sprite has a same sized collision box.

For example if your sprite has 10 animations it will be very hard to ensure that the collision box for every animation and frame is the same.

You might find your sprite getting stuck on walls or appear to float at times.

It's easy to just avoid all of that and make an invisible sprite with the platform behavior and pin your animated sprite to it.