The main use case of functions is that you can parameterize code. This lets you parameterize stylesheets, like set a base font size and primary color, and then derive your whole theme from it, from spacing to secondary border colors to how buttons look when they're hovered.
You can do that with custom properties already. AFAICT functions don't introduce any new abilities on that front (apart from the aforementioned DRY-ing).
Comments
It's not for DRYing/golfing code.
The main use case of functions is that you can parameterize code. This lets you parameterize stylesheets, like set a base font size and primary color, and then derive your whole theme from it, from spacing to secondary border colors to how buttons look when they're hovered.
You can do that with custom properties already. AFAICT functions don't introduce any new abilities on that front (apart from the aforementioned DRY-ing).