Why is the WL charged at Vccp for DRAM cell?
The word line (i.e., the gates of the cell transistors) needs to be charged to VCCP because the transistors wouldn't conduct any current if the corresponding storage capacitor is charged to VCC.

Remember, during the latter part of the read operation, the charges on the storage capacitors get refreshed from the sense amplifiers. The cell transistor must be allowed to pass this refresh charge back into the capacitor, and the only way to guarantee that this can happen for a stored "1" is to make sure all of the gates are at VCC + VTH.