It's simple:

Imprisonment AFTER sentencing is for punishment.

Imprisonment BEFORE sentencing is only to keep the accused available for trial and sentencing, if he either can't make bail or no conceivable bail will be enough to assure he won't take off. Almost no non-lawyer understands this distinction - or they prefer to ignore it.

The real issue is why trial or guilty plea, and then sentencing if there's a conviction, have to take months and months, instead of a few weeks.

Peter