I'll say, for two specific boards:
I am puzzled by devices like "nvidia jetson tegra" or "Snapdragon 835 Mobile Hardware Development Kit" and many many others.
you could actually look this up yourself, but, here we go:
The Snapdragon platform is Qualcomm's processor platform for the smart phone market. It's, like the processor on the Pi, a System-on-Chip (SoC), not only containing a CPU, but also things like graphics processors, and the whole radio / baseband hardware – essentially, Qualcomm makes the chip that makes a lot of midrange/highend phones do anything useful.
Someone has to develop phone hardware. Hence, someone wants a board that you can relaxed attach things like touch screens, cameras … to, with which you can start developing your phone.
Nvidia is a GPU manufacturer. Their main market isn't gaming graphics cards any more, but GPUs for computing. A GPU is very good at doing the same or similar operation on very many data. In 3D Graphics, that'd be e.g. pixels of textures or vertices of some 3D model, but you can use the same hardware to do e.g. climate simulations, protein folding, or neural network machine learning for object recognition in images.
A lot of applications would sound like "if only this small box attached to a camera could detect e.g. pedestrians walking in front of my car". Now, if you have a lot of time, and a beefy PC, then that's no real problem. But in a car, you have neither, and you need something that is very efficient computationally.
So, basically, you need a high-powered GPU and a somewhat OK-powered CPU. And that's what Nvidia sells as Tegra, a SoC with a CPU and a GPU integrated. And there's a development board for that.
I know of multiple companies that use that for real-time signal classification.
In essence, every SoC manufacturer needs to sell chips. People might want to use that chip, but somehow need to a) learn how to use them, and that's easiest by providing an example board, and b) as Edgar Brown says: if you need a working board containing a specific chip, and there is one, economically and reliability engineering-wise it would be stupid to reinvent one.
Also, it's practically unfeasible for a non-digital-hardware-oriented company to develop a high-speed board just to run e.g. the software system that they are actually selling. Just like if you're producing Recreational vehicles (RVs), you really don't need to know how to build cars – you just buy the car base frame from Mercedes, VW, Fiat … and build your RV on top of that, and sell that. People don't pay you to reinvent a modern car, but to build a good RV. The car base itself is just the platform you need to make it a RV; you should be buying that from someone who knows what they're doning.