Worked on a team where every single sprint planning was a useless meeting because we were expected to deliver everything in the backlog every sprint. So what are we really planning?

If you can deliver your entire backlog every sprint, then your backlog is not big enough

Just use kanban

Some kind of agilefall?

If you're gona have to implement backlog items for every sprint then why are they backlog?

Plot twist: Everything in our backlog was bugs (we were a maintenance team).
Therefore PM just kind of assumed we needed to try and fix everything.
Why was a maintenance team using Scrum I hear you ask?
I asked the same question once. I think the answer went to /dev/null.