They (successfully) discourage their voters from voting by mail, then they try to reject as many vote-by-mail ballots as possible. The disenfranchise some of their own voters, but come out ahead.

That said: Please go ahead and vote by mail if that' s convenient for your.

Statistically, the number of ballots the R's can get thrown out are smaller than the number of people who wouldn't make it to the polls on election day, if they'd decided to vote in person.