There are step functions that allow you to approximate it or increment it gradually. I might not know how long an individual step in a process will take, but if I know how many steps there are, I can know approximately how much to increment on the completion of each.

This doesn't suit all usage patterns (like file downloading where you don't know the file size), but I can think of at least a dozen scenarios in which this is perfectly suitable (even if I don't personally love the effect).