Unless you happen to be a person with resources enough to have two or more rigs (no shortage of that around TB), you may think of yourself as 1.) A "big stage, big volume" player (or aspiring to be one, like a lot of younger players -- depends on the type of music or music culture you participate in), and can't even see the need for a capable mid/high end combo (other than maybe a small bedroom practice amp); or 2.) You're a more established older player who never encounters dual full-stack guitar players in their band/gigging routine, and if you had your way, would never need anything larger than a Mesa Walkabout 12 or equivalent (or maybe just In Ear Monitors).
I think appreciating an amp form factor is strictly situational. I do have a variety of amps to choose from (acquired over the years). My favorite setup is my GK head with my two Bag End speakers. Light, great sound, and easy to set up. However, sometimes, depending on the room access or load in/parking situation (and even how fast I have to be ready - time crunch issues, you wouldn't believe...) I'd just as soon have my old Harke 1155 combo (which is on casters, and a very capable amp for nearly all but the largest gigs I have), roll it in, find my place on stage, find the AC, DI, and whatever else, and get on with it. One piece versus two small cabs, a head, and a cart.
So for me, I try to size things up beforehand, and choose what rig to use to fit the occasion. Sometimes the band leader is afraid the band's onstage "look" will overpower the delicate sensibilities of the audience, so he suggests using smaller stuff (and I've learned to diplomatically ignore that and make my own call). It's also my mood too. No denying that I'm a bit fickle on any given gig night.
Whatever you're used to is what you like. If you show up to enough gigs without enough stage horsepower, you want more. If you get aggravated by too many trips and not enough help at load in/out, or you find that the 4x10 speaker you've been using for years is starting to be a drag to squeeze into your hatchback, you eventually try something else. Lots of choices, sometimes too many. It's all got to do with your tolerance, your budget, and your common sense.