Cross Server dungeons/raids had the exact same consequence. In the early expansions you would play with people and get to know them. Sometimes you'd be levelling at a similar pace and cross paths on a few of the instances/quests. Maybe you'd get to chatting?

I know we recruited several people into a casual guild based on doing a few dungeons and getting on well with them. This was, ofc, before guild perks etc gamified that aspect in a shitty fashion.

Ofc you also got to learn people you never wanted to instance with again. Reputation mattered! As the Parent said, may as well be playing with NPCs.