[45ure]

>What's the purpose of the rotary encoder?

The ZMK firmware allows you to assign or map a wide range of keys and functions to it. The inclusion of an EC11 encoder has become a go-to trend, especially in the keyboard building community, and adopted by the established players. However, it is mostly a bit of novelty and redundant after a while and probably only gets used for the obvious media controls.

>I'll buy a USB touchpad and keep it where it rests on a laptop

You can acquire macropads in a plethora of configurations, from a single key to matrix of keys, only limited by the choice of MCU. All built using either traditional [key]switches and/or mix of encoders. The popular firmwares (QMK, KMK or ZMK) allow you to map the afore-mentioned, so you can choose to enter long IP addresses, purpose it for macros e.g Illustrator, Premiere, Blender, Ableton, Autodesk, Eagle etc., or just use it for regular cut-n-paste type jobs via the use of layers.