If everyone can see what I upvote, then I’m going to take that into consideration before voting. If it was private, then I wouldn’t worry about it and vote whenever I want to.
Overall this might be a good thing because it exposes bad behaviour, such as downvoting the person who disagrees with you.
This might be a problem if, for example, there’s a post critical of moderators / admins. You might want to upvote it, but worry about getting banned. If your Lemmy profile can be linked to your real identity, you might worry about real world consequences too.