You’ll pay more in fees if you send a tx that is made up of a higher number of utxo’s.

Ie. Say you buy something for 10,000 sats. It’ll cost less in fees if that tx is sent containing 2 utxo’s of 5000 sats than 10 utxo’s of 1000 sats.

It’s generally smart to consolidate utxo’s when the mempool is empty and fees are low.