They can still vote blank or null. This doesn't answer anything

How makings the vote not mandatory stop these politicians from buying a vote?

From a Brazil perspective, it is a way to guarantee voters' rights for the whole population. The mandatory part is not voting for a candidate, but it is showing up to vote (also, if you can't vote, it is like an R$3 fine). That law and some measures the government takes, like free public transportation for all and holiday on election day, are used to make sure the poorest of the people can vote.

Imagine this, your boss doesn't like who you will vote for, so he creates a job for you to work on the election day. For someone who is living paycheck to paycheck, they will choose work over voting.

In addition, if the vote wasn't mandatory, certain politicians would try to surpres votes. Vote is not mandatory, so why is election day a holiday? Vote is not mandatory, why is election day on a Sunday? Vote is not mandatory, why are we giving free transportation for all? By making an election mandatory, you are weakening those arguments.

In an ideal world, that wouldn't be necessary. However, voter suppression happens, and mandatory vote is a way to avoid this to happen.

Edit: Basically, mandatory vote makes the process more democratic because garantee the people's right to vote.