You’ve forgotten to mention the standard reason for not-obviously-beneficial behaviors: signaling. You are a more valuable ally if you wear the “I Voted” button, you have more credibility when you try to influence people’s voting behavior if you (claim to) vote, and you get to claim membership in the tribe of voters (which is way more fun that the tribe of rationalists who say out loud that voting is irrational).
And like many low-cost things, it’s probably cheaper emotionally as well as being more convincing for some if it’s the truth when you claim to have voted.
You’ve forgotten to mention the standard reason for not-obviously-beneficial behaviors: signaling. You are a more valuable ally if you wear the “I Voted” button, you have more credibility when you try to influence people’s voting behavior if you (claim to) vote, and you get to claim membership in the tribe of voters (which is way more fun that the tribe of rationalists who say out loud that voting is irrational).
And like many low-cost things, it’s probably cheaper emotionally as well as being more convincing for some if it’s the truth when you claim to have voted.