[nine_k]

For some, the physical size is key. Seeed Xiao, Lichee Nano, Beetle, etc fit where e.g. an Arduino Nano won't.

[thamer]

Yes the physical size can be important, and so can the power draw.

Just like you wouldn't use a full-fledged Raspberry Pi for use cases where a Raspberry Pi Nano or other low-power microcontroller is enough.