Well, for me I mostly use my old computers for games, but the company I used to work for used vintage computer peripherals in an actual test & measurement setting!
In the 80's this company bought a commodore 128 and a coco. They used the coco to run a EPROM programmer, and they continued to use this until the mid 90's. When they finally retired it, it was put in the closet. When I was leaving the company, they were also cleaning out some old junk, and they let me take it (And thats how I got my coco!)
The commodore 128 was used because it's serial port is ieee 488, which is also known as GPIB. A lot of industrial control and measurement devices run on a GPIB bus, so the commodore was a logical choice for them. They also used the C128 until the late 90's. (I didn't start the company until 2006, so a lot of this story is second-hand.) They ported their BASIC program to Visual basic, and started running GPIB through a PCI-card. However, their original C128 program printed charts using low-level raw commands through a serial plotter. Instead of printing on a PC's native printer, they decided to port the plotter logic. They kept the old pen plotter, and connected it to their PC through some sloppy adapter they made by splicing up some cables. So, when I joined the company in 2007, they were still printing their charts with this 25 year old pen-plotter hot wired to the back of a 10-year old PC! The PC was running windows 95, but the program was written in a windows 3.x version of VB.