The maker cannot use it - The user cannot see it.

what can it be?

A coffin? idk

lol. that's right.

either that was the most amazing guess ever, or you heard that before.

I don't get it...

people make coffins, but don't use them since they're not dead. the people who use them are already dead so they can't see it.

Isn't it
The people who make it, don't need and that people who have it don't know it ?