I use an IR in my band effect loop - It's really weird to describe but it just tightens the sound up and takes away some of the high end - like Andrei said.
But yeah it's really just a snapshot of what a particular Cab + Mic sounds like - it makes a subtle but noticeable difference to your tone
There ain't nothing subtle about it when we're talking amp heads followed by cabinets in your Reaper FX screen
. The difference is like...planetary.
Basically, how I know IRs are used is the following. When in Reaper, say you want a really cool distorsion. You get a hold of an awesome free VST amp head (you know, the thingy with the bass, mid, treble knobs), you chug it in and then you find out that your sound isn't the face-meltingly awesome one you heard in the demo of the VST.
After you cry a bit, you get to reading and find out that hey, an actual amplifier that you use for your guitar has 2 parts in it. The head, and the speaker, or cabinet. After you've been enlightened and you no longer want to chug beers down, you search for an amp cabinet (LePou LeCab for example), and then you load it into Reaper, after the head. You then notice that that cabinet requires something called an IR, of which there are many readily available for free online. You then load it and finally sound like a normal guitarist with distortion.
This scenario may or may not be exaggerated and based on a true story.