Networking, mostly, in the sense that an article in a high impact journal has a higher probability to be integrated in citations networks. The fact that there is some gate keeping means that it’s valuable to be in rather than out, and that’s something you can use to get a position. Also, better journals (which are not necessarily the highest-impact ones) tend to have more thorough peer review (such as 3 reviewers by default instead of 1 or 2, editors who are not afraid to ask for more reviews if the 3 are not conclusive, etc).

> (btw, I've seen profs delegate to undergrads. And it is quite common for post-docs AND grad students to be reviewers. Trust me, I am one)

I am lucky not to have been in that situation when I was a student, and I did not delegate any further when I got the occasional review from the prof when I was a post-doc. But I am unfortunately not surprised.