Agreed backstay insulator for SSBantenna. These were common many years ago. Still in use some today as the poor mans insulator. These are not designed for high load and come from the household electrical suppliers (if they were marine they would be white lol). They work ok on smaller boats such as the one pictured but then most smaller boats do not have SSB onboard. Not sure what loads they can handle.
If they are in good shape I see no real reason to. It would mean replacing the backstay. Depending on the age of the rig you might want to be doing a close inspection of all the standing rigging. But I would not replace just because you do not need them right off. If it aint broke don't fix it, I am sure you have lots of other things to be working on.