Why does it take power from pcie if it can take power from PSU straight?
What happen if I use unpowered riser?
The point of using a powered riser is to supply directly the power to the GPU, instead of letting the mobo feeding power to your GPU through the riser.
While i never had issues running with unpowered riser in the old days (Bitcoin, litecoin mining with 5850 and 7950), i strongly recommand you to use only powered risers, especially if you are mining with the RX radeon series, which are known for drawing quite a lot of power through the pcie.