Who uses single board computers?
It is far cheaper and faster to design around an already made fully integrated component than to dedicate all the engineering resources to put together elements that are rather standard. 

Many systems require a full-blown CPU, lots of memory, hard drives, and other peripherals along with a tried and tested operating system. Having communication interfaces and video output, although might not be necessary for the application, makes debugging and development much easier. 

I’m currently designing a system in which we needed to integrate a full-blown microprocessor, an FPGA, and some memory. Finding an off-the shelf self-contained and well-tested board for $300 that saved me and my company at least six months of development, engineering, debugging, and production logistics was a total no-brainer. 

Yes, it adds some parts cost to the product, but in our market that is a secondary consideration. Besides, if we needed to, we can revisit the design and integrate the individual elements once we have tested the market and have a mature system instead of just a design idea. 

My company’s products sometimes have more processing power than the computers these are connected to.