Because To know the Slot boundary. From PSS, UE know that PSS transmit at last symbol of subframe. So now it knows the slot boundary. And now UE also knows that, last but one symbol is SSS, now it decode SSS and came to know the CP value. From CP, it comes to know, is it extended CP or normal CP in the subframe.
As we already know the timing of each subframe is 1ms, so it calculates slot, frame, sub frame locations and decode other physical channels info to get synchronisation with network.