Assuming you like beer, then yeah, that's a good analogy. The IRs change how the amps sound. Typically, the IR packs come with mic placements from the edge of the cone to the center with a variety of mics. Placements that are close together might not sound very different, but placements at the end of each spectrum will vary greatly. Also, types of mics will vary greatly.
A while back, there was some variance in the quality of IRs, but for some time now several places have been putting out high quality IRs that you won't find yourself discarding because of the sound quality.
maybe there are more scenarios, but I think most could fall into:
1. wanting a specific tone for a song or their own unique tone.
2. trying to match real world amps by pairing the cab with the amp for a particular sound or just for the most realism they can get.
3. wanting the most variety you can get (using almost any speaker/cab combo with any amp).
4. wanting to create unique setups - pairing speakers you would just never be able to own with classic amps.
it's really all about the sound. Chris has a tutorial on how to sample IRs quickly and easily, so it's not hard to sit down and spin through a couple dozen IRs in no time. It's also a time suck if you keep finding sounds you like and getting inspired to noodle around on each IR you take a liking to.
It can also be a challenge to decide which ones you want to put into the axe (if you get too many). You have to configure each amp to the cabinet, so if you lose your presets, you'd have to set them all back up again. Thus, backing up your presets - perhaps keeping track of your very favorite pairings is a good idea.
While some sets are a tad expensive, you can get your feet wet for $25-$30 on Fractal's site. Just look through the recordings section for ML's posts and you'll see something from every pack he's created (as well as numerous user posts with this or that IR from a cab pack). Hard to go wrong with any of them. I think if you just read the description you'll get a solid idea of if it's what you're looking for.
The Cab (or Amp) factory has a site and you can listen to an excerpt of each IR. Here is a free Mesa one from them:
http://www.theampfactory.com/ultrares-irs/ir-axefx2/
And here is their bundle pack:
http://www.thecabfactory.com/bundles/
The forums have some great feedback of the bundle pack (I think even Cliff chimed in with some glowing comments).
All that said, the fact that IRs change your sound more than just about anything else and it's still not mandatory that you go out and buy more. There are plenty of great IRs in the Axe already. Some from the existing Fractal cab packs.