So you get a few benefits from being a reviewer but it's pretty limited.
One benefit is that you generally get free access to the journal in question for a month or two while you are reviewing. So if you are regularly reviewing papers, you get free access to that journal. If you are part of a big org you probably already have access but it is a nice little benefit for people outside big academic orgs.
You can also include it on your CV if you want and there are a variety of small recognitions you can get for doing it.
And some publishers will also give you a fairly substantial discount (25+%) on texts purchased through that publisher as well as similarly substantial discounts on publishing and editing services provided by the publisher. Sage does this for example and their services include things like translation services, editing, infographic & artwork design, and animating video summaries for papers. They seem like they are quite nice services so getting discounts on them would probably be a good selling point for some people.
Another reason is just because you'd be reading the journal anyways and you don't mind doing a deep dive every once in a while on a random paper in your field. This reason is mostly a "well why not I'm already spending the time" kind of reason but it's a decent justification.
And the main reason is just because it's kind of expected of a lot of PhDs and being good at reviewing is a useful skill to have, especially if you are going to eventually be someone else's advisor or just in general if you want to be able to effectively critique your own works and the works of those around you. Being a reviewer for at least some times means you know the system and you can help keep your own papers and the papers of your colleagues from getting rejected for dumb reasons or oversights.
It's part of the culture. I write a paper. I want to get it published. Journals need someone to review it. If I refuse to peer review others' papers, then in principle people may refuse to review mine and we both lose.
Sure, it'd be nice if publishers passed some of the profits to both authors and reviewers, but that would create other perverse incentives.
> and what real care do they take when they are doing it for free?
Not much. They sometimes sit on it for months before reviewing it.
Pardon my ignorance, I’m very much not an academic, but what the heck do the publishers bring to the table here? Suppose you just published your paper on a free site, and solicited your peer reviewers to annotate a Google Doc or whatever? I can see why journals and their publishers mattered 50 years ago when they were needed to physically publish the information by printing it on paper and distributing it. But I don’t get it now. Why does anyone gift them their papers? To me this sounds like a store where the customers bring in all the merchandise and give it to the store, who then sells it back to other customers. In other words, crazy.
> but what the heck do the publishers bring to the table here?
Name recognition. Top journals are harder to publish into - you (supposedly) need a higher impact piece of work to get published in it.
Same idea with universities. The top ranked universities don't necessarily give you a better education. But that certificate sure helps.
> But I don’t get it now. Why does anyone gift them their papers?
Same answer as above. You're a researcher who is trying to get tenure. You published in Nature. Good chance you'll get tenure. You published on your own site and have a Google Doc of reviewer feedback? Anyone can create that. You won't get tenure.
At least in my field, being on the program committee for top conferences is prestigious and can help with promotion, increases your visibility and profile in the community, is a good networking opportunity with other experts in the field, and for early career researchers provides a valuable perspective on how other experienced researchers evaluate research. The latter is less important in cases where conferences publish all reviews.