I always thought what is this for? Now I found what for. It is for supporting Arabic, Urdu, Hebrew languages. Because those languages are written from right to left. If you set supportsRtl true, your layout will be mirrored from right to left. So you do not have to create special layout for it. If you set false, your layout remain same no matter what language is used.
This is supported from API 17 – Android 4.2.