That's a pretty smart business move by T-Mobile, I didn't know mobile cell towers were a thing
Then there are microcells, which can be privately owned. I worked at a place that had one when I was in mobile. There was a period of time when one of the carriers would sell you one if you were having connectivity issues. It’s possible for instance, living on a hill, to have a cell signal on your roof but not in the rest of the house and they can work as a repeater.
The idea of cellular networks is simple: Put the "source" of the bandwidth near where the people need it.
The idea of CoWs is also simple: It's the same thing, but it's dynamic and flexible.
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There was a time in my life when I was using AT&T as an all-you-can-eat LTE provider through a third party as my home Internet access, because reasons (and hear you me, if DOCSIS had been an option then none of this would have happened).
Armed with a hotspot device that had external antenna connectors, band selection, and a Yagi antenna, I found a cell tower that I thought to be about 14 miles away that had consistently good Internet bandwidth. It was a ton better than several other much-closer towers (some only 1 or two miles away), presumably because it had better backhaul(s).
I made quite a study of things to get that dialed in and working reliably. And it was reliable for months. But then: One day, the signal had turned to shit.
So I did the right thing and I drove over to where I thought that tower was, 14 miles out, to have a peek. And the tower was right where I expected it to be.
But there were men actively working on the tower (with ropes and stuff), and a CoW of much-smaller stature was parked there and providing (rather lesser) backup service.
Which, you know: That explained that.
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They additionally get used some for natural disasters if a tower fails, and also sometimes for other dynamic events like festivals and concerts and such. They're pretty useful when they work, in that a tiny sliver of bandwidth is superior to zero bandwidth. When properly-managed they can reduce contention on neighboring towers so that regular people doing their regular things are less-affected by whatever dynamic event is happening nearby.