This guide explains serial connections between the Circuit Playground Express and other devices. This is very useful for making connections to other microcontrollers or single board computers like the Raspberry Pi. And some sensors require serial communications instead of other protocols like I2C or SPI.
An outbound serial connection can be used for applications like CNC, data logging, debugging code, and many other things.
You can program serial communications with Microsoft MakeCode and CircuitPython - examples will be shown in each.