Those comments might also explain how some cool figure was done

Ehh sometimes you have additional results or insightful remarks that simply don't fit into the page limits. You may want to keep those for yourself and use them for a separate publications rather than give them away.

Well, you don't have page limits on arXiv, though.

This is true, but arXiv submissions are often prepared with a target venue in mind that does have page limits.

Also true, but the arxiv version is often (in my experience) containing the entire paper. Indeed, many conferences ask people to submit the full version to arXiv.

Interesting. I know it frequently happens, but I've never seen a conference explicitly make that request.

Here's an example (ICALP):

> Authors are strongly encouraged to also make full versions of their submissions freely accessible in an on-line repository such as ArXiv, HAL, ECCC.

This is from the call for papers a few years ago. The wording has changed in recent CFPs, due to employing (weak) double-blind reviewing.

They still allow uploading to arXiv (with full names and affiliations) despite being anonymous.

Aye, but in this context "full version" usually means "a version with more detailed proofs/results related to the paper's contributions", rather than "a version with additional contributions".