Twitter is not actually deleting the tweets, only flagging them as misleading. Not all users may read the link taking them to fact-checking pages, and may therefore be misled by potential conspiracy theories. Users in swing states could potentially be more likely to believe such statements.
Other social media sites including Facebook have chosen not to take actions on Trump's posts. Trump posted the same statement about mail-in fraud on Facebook, where it received 170,000 reactions and was shared 17,000 times. Although Facebook's policy is to remove content which misrepresents voting and voter registration, it will not take action against Trump: “We believe that people should be able to have a robust debate about the electoral process, which is why we have crafted our policies to focus on misrepresentations that would interfere with the vote.”