I think the argument is that it gives a more representative parliament and helps fight against voter apathy.

But I'm not sure I buy this. If people are only voting because they have to, the extra votes will come more from people who don't feel they have anything invested in any of the candidates winning, so the extra votes are more likely to be "noise".

Also, voter apathy is a thing. It's a worry that people don't feel engaged with the political system, and aren't moved to vote for any of the candidates. By forcing people to vote, you're simply hiding that. Knowing that only x % of people vote in an election should be a alarm bell to try and engage these potential voters. But if you force people to vote, you don't know how many of them are just voting under duress.