EDIT: I've received some very helpful responses from folks who work at IPR in the comments. Some points:

• Publishers set prices at IPR, and IPR takes a 15% cut, which is smaller than a typical bookstore model.
• I've personally had very good experiences buying directly from indie RPG publishers, but that's not always the case. IPR is providing professional customer service so you don't have to take a risk working directly with the guy who wrote a beautiful little RPG, but can't seem to offer customer service worth a damn. They're out there.
• IPR's slightly-higher-than-usual S&H fees reflect more expensive packaging to better protect their books. I can vouch for this first-hand: they do use better packaging, and it does work.
• drivingblind points out ways to keep S&H-cost-per-book lower... see the comments for details.
• Even folks who work at IPR aren't 100% pleased with the e-commerce software they're using.

The answers to my core question -- what does IPR do for me? -- has a lot of answers, and many of them are publisher-oriented. But the fact that I got so many thoughtful, rational responses from folks who work at IPR indirectly answers the question better than anything. It's clear these guys care about their business and want to serve consumers. They're making a quality effort in a challenging sector of a tough industry, and that deserves some support.

I'm going to leave the post below as originally written, but be advised that my mind has changed.

In my continuing fascination with indie RPGs, I've noted that in the last year or so most indie publishers are moving to Indie Press Revolution to handle their mail-order sales. I don't know a lot about their operating model or the economics of their unique product range, but I do know that as a consumer I don't care for a lot of things I'm seeing at IPR.

First, their prices aren't good. Now, these are indie RPGs, and I accept that I'm paying a premium for buying from small presses. But it seems to me (my memory may be faulty) that I paid less than $22 for my copy of "Dogs in the Vineyard" when I bought it directly from the publisher's web site (which isn't an option anymore, now that the publisher only sells it online through IPR). And it seems to me that IPR is a middleman taking a cut of the money I'm paying... but what am I, as the consumer, getting for my extra dough?

Second, their shipping fees are awful. Perhaps I'm too coddled by Amazon's free shipping deals, but I'll point out that independent sellers through Amazon charge only $3.99 for shipping a single book. IPR charges $5.79 for shipping a copy of "Dogs in the Vineyard," and even more for bigger books. Now, "Dogs" is a great game, but it's a tiny little book. Why so much for shipping? It's only a couple of bucks, but on top of their already high product prices it gets amplified. As a fan of indie RPGs, it's hard for me to feel good about selling my friends on a copy of "Dogs in the Vineyard" when I know it will cost them nearly $28 for a 160-page, digest-sized book. Worse yet, I already own and love "Dogs," but I'm reluctant to buy any games I'm less familiar with from IPR, because I don't feel comfortable with the value I'd be getting.

Finally, the web site is hard to use. It's hard to find stuff, even with the search function when you know exactly what you're looking for. Do a search for "spirit of the century." The game "Spirit of the Century" is the ninth entry on the page! WTF?

Making this worse is IPR's habit of posting a full entry for each available format for a product. There's an entry for the print edition, an entry for the PDF edition, and an entry for an option that includes both. All for the same book. It makes much more sense to me to put up just one entry per product, and show a column with the available buying formats and corresponding prices. It sure would be easier to browse the results of my search if there weren't three nearly identical entries for every project, producing a list so unnecessarily long that it extended over three pages.

I know IPR is scraping out a niche for itself in a hard-to-sell area of an industry that isn't exactly the place to go for profitability to begin with. And they must be doing something for publishers, or so many indie publishers wouldn't be using them. But, as a consumer, I'm not seeing the benefit of using IPR. I never like to be a complainer, especially about a small-fry operation that's serving the gaming community, but IPR needs to do a better job of telling me why I should support them. So far I avoid it as much as I can, but if they continue to be the main outlet for small RPG presses it's going to be hard to follow indie RPGs without them.