The power plant you saw is more a converter station (that is converting voltage and frequencey to suit the railways needs) for the railway.
The converter equipment is normally built on railway cars so they can be transported to a workshop when needed, but which happens very seldom (I do not know, maybe every 5 or 10 or 15 years).
So the switch to the tracks in some such converter stations are used so seldom so it is easier to create a temporary switch only when really needed, instead of having a switch in the mainline track.
But I have seen
a picture in this thread (far down) where it seems to be a permanent switch at Erikstad/Skälebol.
Edit: Or was there a permanent switch, but there was a buffer stop between the switch and the converter station?
In that case I guess that this side track normally functions as a security (if a train will pass from Mellerud to Öxnered, then the switch would force a train coming from Kornsjö to turn right into the side track and into the buffer stop, but not into the converter station).
And when there is a need to use the track to the converter station, then the buffer stop is removed temporarily.