Nope, you don't need it. I believe it's the EVAP purge solenoid on the 88-91's. I've never, ever, ever used it on the many different swapped hondas I've had. I usually cut the wires and hide them in the wire loom with the map sensor wires, so it looks like it wasn't ever there.
If you have to pass emissions, though, you'll need it...other than that there isn't really any reason for you to keep it. If you do remove it, you might as well go ahead and take out your charcoal canister and other EVAP purge related items, and cap off the vacuum line on top of the throttle body and the EVAP purge line on the firewall that goes back to the gas tank.