Yes, so the "computer" would continue running in this state without change --- which is presumably what the machines in charge wanted --- if there's supposed to be a reason which makes stress pleasing to the machines please state it in plain and simple terms (English is not my mother-tongue).
In the original draft, the machines were using human brains as a networked computer. The simulation is necessary because sensory input and simulation is how human brains compute - it's for the humans, not the machines. Unfortunately the studios thought that was too complicated and we wound up with "lol batteries" and none of it makes any sense.
Also, I mean, because without that there wouldn't be a movie.
Yeah - the whole point of the simulation was to keep the brains engaged. A distraction if you will, from the fact that they aren't actually doing anything.