If you are serious, then it is to ensure that the convicted can not benefit from sentence discounting.

For example, if you have two convictions which hold a possible sentence of two years each that is four years.

If you serve both sentences concurrently, that is two years.

If you get time off for good behaviour and reform (you learn to be a cook and run a kitchen) then you could be out in one year on parole to serve your last year through the parole program.