It's like asking why you should write a resume when you won't resumes as part of your job. It serves a similar purpose of trying to get to know who you are.
I thought working at Google or Facebook was supposed to be a pretty chill "rest and vest" situation, esp at the former (?)
Remember, with the kinds of education people have at these companies coupled with the amounts of money they are receiving, "the bullied become the bullies" is a serious occupational hazard.
I think this is only true if you want to work on something interesting. If you're willing to work in some obscure corner of ad infrastructure or Google Surveys or something and you're not trying to get promoted, you can stick around for a long time at Google and not get much done. At a big organization like Google the pace is pretty slow even if you are trying to get some work accomplished, so that provides cover for those who aren't as effective.
Ha ha, fair enough!
Usually a good manager can spot pitfall projects from a mile away and put you on a good path to a promotion. But only being good is not enough, you have to work on the right projects, have the right results and the right visibility. If those variables are not there you may go through a downward spiral that ends up in a PIP.
For me, leetcode is basically just what is taught at university level. So they're hiring people based on if they know stuff they would learn and forget from university.
The thing is, it's not about having free time, it's about being so dedicated to getting the job that you'll spend ages learning things you would never do during your day job and if you did do them during your day job you would be considered a cowboy because there are predefined libraries that are optimised to do those things.
For me, the main reason I don't want to work for FAANG is they have a reputation for long hours and that the hardest thing at the companies is the interview process.