Checkmarks verify that the post is not made by someone claiming to represent a person, who does not actually represent that person. It solves the issue of fake accounts, which is a very real problem. For politicians, it doesn't really make much of a difference anyway whether they are the person physically typing the message. When a politician gives a speech, that speech will be written by a team of people working for the campaign. Even when answering questions at a town hall, the answers will just be talking points decided by the campaign, and I think that's common knowledge. Even if Twitter could verify 100% that a certain person typed the characters what would that get us? Gravel would just ask these kids what he should type, or they would send him the text directly. You could argue that this would at least guarantee he's seen the posts and that he therefore approves them, but don't we know that already? If he doesn't approve the posts he can either change his password or ask Twitter to remove the accounts verification.