Some people's preference is literally "I don't care"
Other people may have a preference on a specific issue that favors a single candidate while other issues better favor another candidate.
It also reduces actually researching candidates to a waste of time. If I take the time to carefully research 5 candidates and their stance on all of the issues, weigh them carefully, maybe even attend a campaign event to get clarification...and then another guy walks up without having any idea what he's voting for that votes for the other guy...his vote counts exactly the same except that he's only there because he has to be.
I mean, that's democracy. The point isn't a perfect system where everyone is making completely informed decisions. The goal is to keep public officials accountable to the people and their interests, however uninformed.
Diluting the voter pool so much that the people paying enough attention to hold a candidate accountable can't actually have an impact?
I happen to know that Brazil has mandatory voting for most people (it’s optional for 16 and 17 year olds, and for anyone older than 65).
They allow people to vote for nobody, and to vote for “whoever wins.” Since Brazil requires an actual majority vote, voting for “the winner” amounts to “I want to avoid a run-off.”
The main preference this may reveal is a preference to avoid being fined.
It also makes elections a more accurate reflection of everyone's preferences, instead of just the subset of people who are motivated to vote and able to get to a polling place.