Apparently Justin Timberlake has some influence in his native Tennessee. Justin posted a selfie while in the booth on his social media page during the last election. Unfortunately, he was forced to take it down because at that time in Tennessee, it was illegal to post any pictures from inside the booths. However, Tennessee recently made it legal to do so. As long as you do not take a picture with the intent of committing voter fraud or for vote buying, it is totally fine to do so (in Tennessee at least).
Many states do not allow pictures or video in the poll booth.
I could see the benefit in being able to do so. I think it could help with attracting more voters, particularly younger voters. I don’t think it has to necessarily be a celebrity to be effective. Young people will want to go out and vote the more they see their peers doing so. Some might go out and vote just so they can take a selfie. Either way, I think it brings attention to the subject. I don’t think you should be able to post WHO you voted for, as many people might vote for someone just because a celebrity that they follow voted for that person. But I don’t see the harm is taking a quick pic on your way out of the booth.