Why is it important to wear a sports bra after explant?

Your implants will have created a pocket in your chest in which they sit. Once the implants are removed, then this pocket is empty and therefore creates a space in which fluid could accumulate (seroma). Wearing a sports bra will help to provide compression on your chest in order to reduce the risk of this fluid building up in the space.

It probably isn't important to wear a bra of any type after a simple implant explant. If it was a simple explant, only the incision needs to heal. The worry is that blood or fluid will build up in the space where the implant was inside the capsule and the compression of the bra will help prevent this. In my experience it doesn't seem to make any difference and even if fluid where to build up (blood is highly unlikely in a simple removal without doing anything to the capsule), it's very easy to drain off with no implant there to worry about. I recommend wearing the bra for a week after explant but don't require it.