Here are a few places that I've found pi's:

Rapid prototyping: I created a PoC to take a webcam snapshot every half hour and upload it to a server in an afternoon. Freelance project. Could it have been done with a microcontroller? Yes, but not in 4 hours.

Local digital displays in a gym: we built a system with a number of overhead 60" TVs, each with a pi on a VESA mount to show scheduling and workout information in a gym for a client.

HVAC controller: bid on a project where the customer's original concept was a Pi managing a rooftop HVAC system for large buildings. They outgrew the pi and wanted a new solution.

Data aggregator: collecting sensor information via BLE (bluetooth) and uploading to an internet server over Ethernet.

Remember that "cheap" to a consumer doesn't have the same definition for a business. To most profitable businesses, a $100 computer that fits on the back of a TV and consumes a fraction of its power is cheap. In fact, one of the reasons that Pi's were so hard to find for a while is that the Raspberry Pi foundation was prioritizing industrial/commercial customers over hobbyists.