The copper mines of Anglesey were huge by anyone's standards, not surprising if the Romans wanted to take the operation over. Hard to say whether getting rid of the Druids was a by-product or the real underlying reason for invading Anglesey.
I'd have thought displacing the ruling caste quite high on their shopping list. The Romans weren't very successful further south in Cornwall though, tin being rarer it's arguably of far greater interest; officially they didn't get further than Exeter or thereabouts.