I don't think it's so much about wanting a "specific" combination of microphones, speakers, and placement as it is getting a variety of tones.
Different microphones, mixes of microphones, and microphone placement all change the tone of a particular cabinet and speaker very drastically. SM57's for instance are kind of bright with a lot of midrange while R121's are darker and smoother but have much more low end. Mixing those mics can get the "best of both worlds" (or if done improperly can sound really bad). And if a mic is closer to the center of the speaker, it will be much brighter while moving towards the edge it will be much darker.
Factory cabs have one (sometimes two) selections for a given tone for a particular guitar cabinet and speaker. If that combination is what you are looking for, perfect! And there are sometimes multiple brands which gives different options for that cabinet and speaker. So there is a wide variety in the factory cabs.
But with a cabinet pack, you get a lot more tonal options for a particular cabinet and speaker (sometimes a few speakers). Typically, you'll get several different microphones, microphone mixes, and mic placement for each microphone. So you can really hone in on the sound you are looking for.
There are a lot of options in those packs. I checked and my ML Bulb Zilla has over 300 impulses and my Ownhammer 4x12 Checkerboard (from their website) has over 600. But both have included folders with their favorite impulses for each microphone and their favorite mixes to simplify things.
I have an interesting thread in the cab section about IR's and a lot of people say they find one IR (or a combination in stereo) to play with ALL their amps. Michael Britt apparently shoots all his profiles through the same speaker and cabinet. I think that makes a lot of sense from a live standpoint.
Personally, I'd rather have a single good cabinet pack over the factory options. If I'm going to settle in with one or a few IR's, I want an IR with a wide range of fidelity that doesn't have so much character that it only sounds good with one amp. So my favorite combos are, for ML the Zilla V30 Mix and Zilla Greenback M Mix (from the Bulb Zilla pack), and for Ownhammer the Marshall Greenback M75 OH1F mix and the Mesa Recto V30 OH1F mix (from the Core Tone bundle). That mix of a Greenback and V30 is really versatile, and you have IMO a smoother sound with more high end (if you need it) than stock cabs.