Hi Poose,
the intention of the warning is to warn a crew in flight that the reverser is unlocked as opposed to warning them before departure?
It's purpose is to warn crews that it is not locked in the fwd thrust position both on the ground & in flight. It could be very messy if you tried to take off with one unlocked and you didn't know about it until airborne.
It can sometimes be cleared by selecting reverse idle with the engine running, then selecting fwd idle to let the stowing logic run.