OpenArena's mainly inspired from the existence of
FreeDoom so there's that. The relation is both have rather punny double-meaning names and both are replacement data for id's most popular shooters. Freedoom can be played online in a totally Free way with
Odamex.
Some of the motivation to develop OA also came from playing
BZFlag online when that was popular, and was the most popular online shooter game from the universe repositories at the time. I don't know how many players it has today, and certainly had none of the maps in their current gallery when I played
Fast forward to 2014 there's lots of arena shooter choices with wildly varying licenses and now OA is just seen as saturation-adding "another pointless arena shooter" among them