It’s so the votes can be counted overnight and the announcement made on the Friday. Then the incoming government sorts things out over the weekend and we can have a new functioning government on Monday.
If they did the election on say a Monday then the announcement would be made on Tuesday, then the country would just be in limbo for 2 days while everyone tried to work out what was going to happen. This way, the limbo happens over the weekend where not much business needs to be done anyway.