Reasons to change smites on warwick?

Challenging smite is probably the most picked smite for ww and it seems like the one that has the most synergy with his kit. What are the reasons why blue or purple smite are taken some times (especially in high elo games where they have a reason and no just random decision)? It seems like he wouldn't need the sustain from purple, but maybe it speeds up his clear w/ the aoe? Blue I could see as a ganking tool, but it seems like he's so strong w/ ult ganks it makes the most sense to just go w/ it and bring red smite for more strength when R is up rather than having blue to help when R is down.