It's an indirect measure. The target of the measure is not to prohibit being outside (although that is the first-order effect), but to prevent mixing households.

The real reason is that you're not allowed to coop up with guests in your home, but the police is not allowed to enter private residences and enforce that. So the night-time curfew is now being floated as the one thing that they can enforce to discourage people spending their evening with friends. Of course, it does nothing to discourage day-time visits, and I suspect evenings-with-friends will now turn into adult sleepover parties, but I guess we're still big on symbolism.

In my opinion, it'd be better if the government just declared the battle against Covid lost. They've tried for a year to appeal to the better nature of our citizens, and those citizens have democratically elected not to care about Covid. But at the same time, I know that openly tossing the towel will only lead to more people ignoring the measures.