I just put it in a line under my “service” section, (e.g Reviewer: conference1 (year), conference2 (year)) most other grad students I know do the same. When you’re at a more senior level it might not matter as much anymore though.

You get to put it on your CV. Probably see some cool papers before they come out. And train your discriminator as the other comment says

I also did quite a bit of reviewing this year despite being new to ML, I think reviewing 1-2 papers is good for a new person but the marginal benefit decreases sharply when you review more