I highly doubt most programs know when they'll finish, usually there's multiple stages of say installation, copying files, editing the registry, generating some other files, downloading updates, etc. Some of those stages take milliseconds, others can take minutes or longer. Maybe some take seconds on an NVME SSD but takes 4 minutes on an HDD (verifying files commonly has this behaviour), etc.

If the progress bar doesn't differentiate which stage it's in then it's purpose is mostly just to let the user know it's doing something.

I had an Epson scanner that had a progress bar while scanning. You could HEAR the scanner progress, and it wasn't close to even - sometimes it would stop for a while, or even back up. It didn't matter though, the progress bar kept an even pace and always finished at the exact instant the scan was done.

That has always stood out as one of the more impressive engineering feats I've seen.