The web interface is primarily intended for account administrators to keep an eye on overall mail volume, add domain-wide senders to be exempted from spam filtering, and pay their bills. The API is designed to handle virtually all updates to email forwarding routes.
That being said, it is possible to use the web iterface add new alias->forwarding pairs, edit forwarding addresses or delete existing aliases. If you have a need to add or modify only a few users, you may wish to use the web interface.