I love technology and the modeling this IS really cool, but in many ways for the kind of stuff that many of us do it seems like going the long way around to get the same result?
You're going to get to maybe 90 or 95% of the way to your desired tonal goal by sending from a mic or from a decent DI either pre or post preamp depending on what you want with your rig of choice. Usually for a fraction of the cost and/or setup effort (going in and sampling and that whole process). And in the end I always end up feeling that the end tone and feel of modeled stuff is 90-95% of the sound/feel you want anyway.
Its the same when I was looking for a small/cheap board with at least 16XLR ins for smaller self-PA'ed gigs and some recording and someone pointed to a digital board. Really cool and sweet things that you can do with it, but sometimes I don't want to get 3 clicks into a menu to adjust an EQ frequency when we're on a quick setup, even if its a limited 4 band or a 3 band with semi-parametric, I'd rather twist a couple knobs. and also have an outboard 31 band EQ for the room.
If you're asking "why model vs. just bringing a DI or mic'ing a rig, because it's really kinda the same thing," there are a couple of reasons to consider modeling. Done correctly, with the right interfaces and with the right software/app, you can get studio quality tone and the malleability of having a boatload of modeled gear at your fingertips. Literally.
But that seems to be the fundamental misunderstanding of what's in a good modeled rig, and exactly how malleable the apps are these days, as well as how user-friendly the software interfaces have become, both with WiFi and Bluetooth integration as well as wired MIDI controllers. But the key ingredient is a smaller footprint and much better stage volume control, along with an IEM mix that goes with you wherever you are.
In my instance, I run mine via my iOS device, but others run theirs via their MacBooks (my guitarist just switched his whole rig over). What you get by doing that isn't 3 clicks into a menu. If I need to change something on my rig (rarely happens, but let's say I do), I tap the specific item I need to adjust, and twist the knobs necessary to adjust the parameters. My modeled amp needs more bottom end? I change the settings on my HPF to allow for that. I need a cab or mic sim that is a bit less boomy? I tap the cabinet and it displays not only which cabinet I have, but a scrollable list of other cabinets, mic option, mic distances, and in Mac OS run environments with some software, dual mic capabilities.
Essentially, I pack a whole lot of tonal control and variability into that rig, because I can bring a modeled SVT, or a B15 or an 800RB, a load of compressors or fuzzes or distortions, etc., into one small device. And the models are surprisingly close to the real thing.
And then there's the physical interfaces one must compare in terms of mics and preamps and DIs. My guitarist and I both use Focusrite products, mine is an iTrack Solo and his is a Scarlett 212i. Insanely good, studio quality preamps, and very fast, high quality 24bit 96kHz processing. In mine, that gets sent to an ART Cleanbox Pro 2-channel DI. One channel is sent to the board, one receives from the board and patches to my IEM rig, which encompasses a compressor/noise gate, EQ, BBE Sonic Maximizer and then to my IEM transmitter.
My entire rig, which encompasses my iOS rig, my wireless, my DI send and DI returns, return processing and IEM transmitter all sit in one 8 rack space box and weigh about 32 pounds. And out front, it honestly sounds like 3200 pounds of gear.