Yes. It's placebo.
Remember that progress in human perception is not a number of files copied or requests sent, it is rather time. Of course for some Users it will be that machine-related thing, though so it depends on the context and background User may have.
When it comes to time very often (I'd say even: usually) telling the exact progress in time is not possible – which is especially valid for multi-stage processes, consisting of different operations and with multiple dependencies that may affect it. Therefore, the progress bar with constant length needs to represent variable time.
From now on, everything comes to building up the satisfaction of the User. Various techniques can be used here, for example:
- encapsulating some of the processes in a seemingly not changing progress indicator, only to let it quickly straight go to the end as most operations are done,
- using an additional pattern to show the process is alive (a blink on the progress bar (e.g. Windows 98 loading "progress" bar used only this),
- faking that something loads quickly (see Safari mobile load indicator – which is mostly a fake, it goes to 90% and then slows down if the page has not loaded yet).