Single board computers have GPIO and interfaces like SPI and I2C. They also tend to have lower power consumption and can run from 5 volts. If you want to interface with low level hardware or run from batteries, the SBC will usually be the better choice.

I run Pi-hole and PiVPN on my Pi 0W.

Media server client, pihole, emulation, programming or home automation project. You could even prop it up as a standalone web server and make some kinda creative thing.

If you’re interested in home automation, I think that there’s a reasonable argument for running it on separate hardware. Not much by way of hardware requirements, but you don’t want to take it down, especially if it’s doing things like lighting control.

Same sort of idea for some data-logging systems, like weather stations or ADS-B receivers.

Other than that, though, I’d probably avoid running an extra system just because I have hardware. More power usage, heat, and maintenance.

EDIT: Maybe hook it up to a power management device, if you don’t have that set up, so that you can power-cycle your other hardware remotely.

Donate it.

Using it just draws more power.

Run Vodafone’s Dreamlab on it and donate CPU cycles to research.