Why is Freeze-thaw weathering vital when a glacier is going to erode its valley?

Freeze-thaw weathering has nothing to do with glacial erosion at all. It is a process associated with temperature fluctuations causing the freezing and melting of water to cause expansion of cracks in rocks that eventually split them apart. FTw does occur where glaciers are present but glaciers have vey different erosiion principles and they are resposible for the creation of the typical postglaciation U shaped valley NOT FTw.