Understand the Steam Controller came out 10 years ago and was meant to be used in the decade or two prior to that when “real PC games” didn’t support gamepads. Contrast that with today where CRPGs and RTSes often have official bindings.

There are two ways to use a trackpad. The first is to swipe (like a laptop trackpad) and the second is press and hold. For the former, the delta between where your thumb is and where it was is used to translate to cursor movement. For the latter, think of it like an analog stick. The center of the trackpad is 0 and your input is the delta between 0 and the location of your thumb at this moment.

So press and hold lets it emulate an analog stick and swiping is very useful for moving a cursor on the screen. And there are/were plenty of ways to switch between the modes on the fly.