Every paper is a different white.
Yes, and that's why it's so useful to keep several calibration targets for different papers and scenarios. Not all calibration software lets you switch between them easily, though.
But even so, you can with little effort hit a "generic" paper color that is
way better than simply punching in D65 and some arbitrary brightness value that someone says will work.
If you don't print, paper white is still the only realistic reference. Some advocate D65 in this case, but I don't agree with that, mostly because different sensor/panel combinations will produce different visual results. Everyone who has tried to match two different make/model monitors can confirm this. You need different numbers to match them.
Targeting paper effectively bypasses this potential problem.