"Mining" means adding transactions to the blockchain. Ad absurdum, if no one transacted bitcoin, there would be nothing to mine. And contrary to what some claim, given Bitcoin's fixed block size and a few other constrained parameters, there is a very clear (and enormous) electrical power cost per transaction (with some range given that transactions are not fixed size - the more participants in a single transaction, and the more metadata, the higher the power cost per transaction).

Mining means finding the hash for the next block. Mining empty blocks if there are no transactions is perfectly fine. The expected size of a block makes no difference to power consumption. The only relevant parameter is the point at which it becomes unprofitable. You could argue that if less fees were spent, some barely profitable mining rigs would be turned off in anticipation. However, Bitcoin is not at the point were fees are decisive yet.

The transactions add value to the network which incentivises mining.

The purpose of mining is to confirm transactions.