I find all 4 options useful. I use them constantly.
I use them for that, maybe someone has more uses for them.
- Need Ammo: Useful to know someone need ammo so you can let them kill someone on melee or to know why they are not shooting.
- Come here: Obvious reasons, I also use this one to mark if the antagonist is on me.
- Need help: That one is really useful, and IMO should be used more. I have been saved and I saved people that has used it when wounded.
- OK: probably the less useful, only feedback or as "gg"
I second the use of a chat in the lobby, in game I think is different because you have cross-platform and it's hard or them to use a chat while playing.