Thanks Jon & Carol .

"Why were cup marks engraved " ?

Probably lots of different reasons rather than one over arching explanation .
e.g. Apotropaism , decoration , humanising the landscape , part of a communication with the stone , self , or spirits/gods etc ,if the latter possibly related to appeals for increase /decrease in fertility , rain , drought , luck etc , territorial marker, route marker etc .
Worth pointing out that one cup mark is sometimes the sole component and there are marked rocks with no cup marks .