Sure, in the movie it mainly functioned as bait/a trap for the Rebel fleet, but presumably it would've been used after that, especially when the rest of it was completed.

However, in Rise of Skywalker we have the Final Order, a massive fleet of Star Destroyers which have close to the same firepower on every single ship (although we never see them use the big cannons on the Rebel ships). A recent issue of Darth Vader revealed that Palpatine had these ships around since at least around the time of ESB (Which also kind of explains their retro look).

Then again, even 30 years later Palpatine says the Might of the final order will soon be ready (Implying it's not quite there yet), so I'm guessing it's not yet complete/installed by the time of ROTJ. So Maybe DSII would've served their purpose in the interim.