When you use the force mod load to load a client-only mod on a dedicated server the server isn't flagged as modded when it really is.   This is against the clients who join a server looking for a nonmodded experience to get slapped with a modded experience by false flagging.   By default the server won't load client-only mods and wants a server-only mod so that it may report that it is in fact modded, and this goes around that.