what do you do with them?

Personally, I just finished up a project with one to poll outdoor and indoor weather sensors and notify the household over discord when it's warm enough outside to probably need air conditioning, when it's cooled off enough to probably not need AC, when the AC goes on, and when the AC goes off. It also parses and displays and notifies information from the National Weather Service when there are relevant alerts in my area.

Cheap MCU dev kits in general, for me?

A somewhat-elaborate fan speed controller for a wonderful old pro audio power amp that also needed a good bit of other rehab

A window fan controller, for summer use, that only turns on the fan when it's cooler outside than it is inside -- so it only cools the indoors, seeks never to heat it

A sensor for automating the color temperature of my indoor lighting based on outdoor conditions

A macro pad that ultimately works kind of like a Streamdeck, but without the built-in display features (that was a download-and-print project)

Other random stuff, like an ESPHome widget that talked to a window aircon via infrared commands (mostly just a download-and-run project)

They're just cheap little programmable low[ish]-power computers on a board, with some flash storage and RAM, and a fair bit of low-level IO. Often with wireless network connectivity as a built-in. They do stuff. :)

If the code is tight-enough, they can do a remarkable amount of that stuff -- in terms of raw compute, they present something near-ish to the Pentium era of personal computing for a few dollars.

For me, they make sense for simple projects. When something like a disused laptop or a Raspberry Pi-like device [and the accoutrements, like a whole-ass operating system] seems like overkill, then: It's probably overkill.

With a dev board, I can breadboard a circuit and maybe even leave it that way forever, or get a custom PCB drawn up with KiCad and made inexpensively by JLC PCB or Osh Park or whatever, or just solder wires and parts to the dev board like a caveman.

All methods work, and the cost of screwing it up is usually very low.

After that, the thing gets powered up with one of the featureless USB wall warts that we all have accumulated too many of and it just sits there and does its job.

(And honestly, these Pi Pico and random ESP32 dev boards are also overkill for what I do with them. Most of what I've done could be accomplished with an MCU chip that costs pennies instead of dollars, but my quantities are low-enough that I just don't care much about a few dollars for occasional one-off projects.)

I am currently working on a Bluetooth Mesh home network allowing me to control lights, gather sensor data from various places, etc (think interfacing with a home alarm system for presence detection).

Also, a home air quality monitor, both with a display and reporting to my mesh network.

Or you could build a "plane radar" like this one :-) https://github.com/ironicbadger/ESP32-Plane-Radar

Similar class of chip, but I use the rp2350 to perform computer vision with LiDAR to detect when people leave dishes in a sink and beep angrily at them.