Beta is the period that the "Server Owner" with the "Community" test a few things fix some bugs and so on.
It's an important period before the launch I think. So, ye it's very good that some servers use that before they open. Imagine to open without "Beta Testers" and when it's launched to have tons of bugs people cry people leave simple as that.
Even big companies like CD PROJEKT, or DICE (Creators, of Cyberpunk2077 and Battlefield 2042) they fucked up and now they are running like hell to build an updated patch to fix all the damage because they didn't test or because they rushed it.