The newest ESP32 can run Linux and it's getting close to a Raspberry Pi

Stated another way: An MCU, like the ESP32, is a tiny computer, sans the hardware you physically interface with it.

The computing power, and memory is much lower than your desktop or laptop PC (or mobile phone), but if you are using it to run a dedicated task, instead of using a big OS like Linux or Windows, it can complete the tasks really fast (often microseconds) and with minimal power use. This is because it's easy to program to do exactly what you need, with nothing competing for the hardware.

A note on ESP in particular compared to other MCUs: It's one of the only (Or was?) options which has Wi-Fi integrated into the MCU itself. It's a good default if you want that.

They're generally intended to run flashed firmware and be single-purpose. They usually come bundled with peripheral components and typically have breakouts to add more (pots, sensors, etc).

They're great for doing one thing (though some support firmware switching), and you can build the 'software' for them pretty easily using Python or C-like variants.

So for hobbyists they're perfect. You want a quick LED christmas display you can toggle between Rudolph and Santa from a quick network call? You want a humidity sensor in your bedroom? You want to create a networked security camera? These are the types of use cases for MCUs.

If you want a web API for something in the physical world, make ESP32 do it.

Smart homes are dumb unless you are presence aware. Looking at your phone to do a thing is just moving a light switch to your hand.

The Who is — does the automation make sense for one or many people. If two people are in a room what happens? If one person leaves what happens?

Esp32’s are pivotal in actually making a smart home predict what you want making the home more of a friend, “get on up there Peter Townsend.”