[jkukul]

With server side tracking you're not able to identify and properly track non-logged in users. GA (and other client-side tools) take care of this, via cookies.

Additionally, a common argument is that the server side logs contain a lot of logs from bots/crawlers and GA (and alike) can filter them. The other side of the coin is that GA (and alike) are not able to track users with Adblockers.

EDIT: not sure why I'm downvoted - the OP asked for some reasons why people use client side tracking and I listed them. I didn't say that I support these practices, but maybe I should have made that explicit to comply with the overall sentiment of this site.

[kalleboo]

For the vast majority of sites I would bet using IP address alone would be enough to track enough users individually over their single sessions to be statistically useful.

Does anyone have any experience? How bad does CG-NAT mess this up over a large enough cohort of users?

[Arch-TK]

You can set a tracking cookie when the user first accesses your website without needing them to log in.

[bombolo]

They also ensure that you need the annoying popup to consent to tracking to comply to the law, rather than doing no tracking and not annoying your users.