See my sibling comment. Getting your name checked off is compulsory but nothing stops you from handing in a blank ballot.
For the purposes of this comparison, those "informal" votes still count in the typically used participation statistics. Voters intentionally case "wasted" ballots in other countries too.
Why would you hand blank ballot at. That point? You might as well vote.
"I don't like any of the rat-bastards." "I don't care." "I think it's funnier to draw a dick. (And I don't care.)" "I trust other people to make the right choice." "I refuse to participate in this bourgeois sham election." ...are all reasons I've heard, even if I don't actually understand any of them.