Can any one tell me WHY....both WESTMINISTER ABBEY & ST PAULS..have..?

CLOCKS in thier west towers...! but not on thier right...is thier any political or relegious signifigance to it..! and why are the unfilled towers not in thier circles ..not have any monument or chiseled effects on them...is it one reason that..to do with grenwich meantime? before its conception...and after..?