Wi-Fi on hardwired iMac?
Without special software or hardware to do so, multiple connections are not combined for increased throughput, knowing as bonding. The connections are utilized in the order shown in System Preferences > Network (screenshot below). You can change the order by clicking the gear icon > Set Service Order.

There is no harm in letting the computer connect to the router, other than possibly a slight impact on load in the router - but assuming the connection is not being actively utilized, impact should be slight to none. The only way I am aware of to prevent the Mac from connecting to the router is to not tell it the password or turn Wi-Fi off.