No one cares. It's publications, grants, and patents.
Why do think anyone does it then?
- friendship with the editors, who have asked them to do the review
- belief that providing peer review for a journal helps weigh the scales in their favor when publishing in the journal
- or even be a path to becoming an editor for the journal
- a way to keep up with the field
- they love to criticize and nitpick, and peer-review comments are more likely to get a reaction than HN
- desire to be a gatekeeper
Those are the ones I've come up with on the top of my head. I'm sure there are others.
While dbcooper's comment mentions "No one cares", I believe that's specifically that employers don't care.
I believe that because dbcooper's comment was in response to your previous comment 'When you apply for a promotion and they ask ...' and your earlier 'If you're employed as a researcher...'.
No one disagrees with the anodyne statement that "people care". I know people who are not employed (eg, retired) and do peer review even though there is no possible career advancement involved.