I'm not sure of the exact reasoning. I would hazard a guess at cutting down on staff at night, but don't know if that's true.
As far as I know it is a permanent thing. They are allowed to do it subject to some restrictions, such as not doing SRA approaches.
This sort of thing is not unusual, either through being retained on the approach frequency who will perform the function of the tower also, or band boxing the frequencies so aircraft will still change from approach/radar to tower but they will still be speaking to the same ATCO.