How to think about performance evaluations as a manager or individual contributor
Having been on both sides of the table, performance evaluations are something I've thought a lot about...
1) To what degree you should take them seriously or not depends a lot on your manager and company.
Some managers place a lot of emphasis on them, but others treat it as a check the box. If you have a good relationship with your manager and they provide consistent feedback on a weekly and monthly basis and indicate that performance evaluations aren't a big priority, it's usually something you don't need to focus on.
That said, if there are any issues or concerns, then you will want to watch the situation more closely...
For example, performance evaluations are sometimes used by managers to move someone they don't like out of a role, team, or company (here, ageism, sexism, and racism can and do play a role)...so it's important to notice if a manager starts treating you differently leading up to one.
I also recommend keeping a doc (do not do it on a company computer/resources) with kudos/achievements and any issues or concerns you notice. It can be as simple as a timeline with date/title/short description.
This is a good habit to get in even if your current manager is easy to work with because they may change, or you may get a new manager or team.
Some companies also treat performance evaluations quite seriously, but in most cases, it is more of a benchmarking exercise that provides metrics to the executive level and justifies bonuses, unit/team performance, etc.
2) Because performance evals vary widely, it's important to practice detaching yourself a bit from the process.
If the company doesn't take them seriously, or the manager doesn't, you may get feedback that's careless and in a hurry.
If your manager has an "oh s***, I need to finish this" moment the week of your performance evaluation due date, how thoughtful and useful will that feedback be?
I don't say this to suggest they can't be done well... because some managers can and do care about them and prepare, but in my experience, that is 50/50, for various reasons, but especially because managers often receive little coaching or training on how to do them well and intentionally.
The important thing is to notice and be realistic about what they mean in the context of your company/organization. Particularly in large corporations, they can vary widely by team and business unit.