Exactly, its a way to keep out spam from anything that builds on top of Nostr.

That can be the a setting in the chat client to restrict replies, private DMs, etc. Other apps that rely on messaging can use PoW to keep out spam too, like blog comments ( https://damus.io/log/2022-08-02-introducing-damus-log.html ), forums, captchas etc.

Facebook (hah) commnunities where you want to welcome people, but the group admins have no time or interest to deal with bots.

Basically anywhere there is spam, a simple PoW setting (only show above difficulty X) can be effective.