What should you talk about with your manager in your compensation discussion session?

You are right, there is literally nothing a big tech manager can do, at the time of compensation discussion, to change your compensation package. However, these are not completely meaningless if you realize what's going on behind the hood during compensation planning process and ask intelligent questions!

In general, the comp planning process for you in big tech goes like this:

1. Everyone's performance rating is finalized.
2. Based on your rating/your level/your location, the company compensation model will output specific numbers.
3. Depending on the company/organization, your manager then can make adjustments to your numbers (salary/bonus/stock typically). They are typically given a "budget" to work with across their whole team. Generally, each of these adjustments needs to be "justified" with a rationale and there's a limit to how much managers can adjust.
4. These numbers (and the adjustments) then roll up the reporting chain w/ each higher level getting to make further adjustments with generally bigger budgets. Sometimes, higher level directors can reset all adjustments front-line managers have made.

In general, you can ask these following questions (your manager may or may not be give you a straight answer, but some may, so you might as well ask).

1. What was modeled based on my performance, and what was adjusted for me and why?
2. How much discretionary budget was available? How does this compare to prior years?
3. How much does my location/level/rating have to do w/ my compensation changes? How much higher would've it been if I earned XYZ rating or gotten promoted or changed location?

It is ultimately informational. But these could be useful information.

For example, a common adjustment rationale is "Retain Key Talent" alongside a small bump in your RSUs, which could mean that your team/org/product values your as a key contributor compared to your peers and willing to go the extra mile to make sure you are compensated above what the model gave you. Adjustment can be downward as well, and they could signal that perhaps you just barely got the rating you ended with.

One last tip, now that I'm on the "dark" side - please be empathetic to your managers during comp discussions. Especially during bad economic times, comp increases can be little, so these discussions can be difficult for both you and your manager. The budgets managers have typically are small compared one person's compensation, and they get even smaller during bad times. Overall, managers are just mostly messengers for the company "model" when it comes to comp, and so please don't shoot the messenger!

P.S. Feel free to have discussions about your compensation outside the regular comp discussion window. Most managers won't mind.

P.S.S. If you work at Google, one of the quirks of the system is that managers have no visibility into your initial join grant, which means that if you are facing a 4th year cliff, they have no way of knowing until you let them know. Perhaps this is true elsewhere too. Of course, telling your manager about it doesn't mean they will be able to make adjustments, but still.

Based on the context you gave, this meeting seems largely useless, but I do think it has value for 1 particular thing: Acting as a natural touch point to follow up on your promotion discussion

Promotion is the core way you increase your compensation, so this seems like a reasonable meeting to bring it up with your manager and have some amount of a serious conversation about the gaps you have to get to the next level. Here's a good video covering how to approach that conversation: Asking For A Promotion: The Wrong Way vs. The Right Way