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

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.