It's not just deciding the candidates. It's a lot less of a vote in an election (like a primary) and more of a neighborhood meeting. People give speeches, express support or disdain, and try to convince other people to support their candidate. I believe they still vote at the end, but it's not like an election where voting is the only thing you do. It's different depending on the state, but that's how it generally works in Iowa.
Edit; also important to note that caucus has different meanings depending on where it happens. I.e. if it happens in Congress then it's a meeting of peers with similar backgrounds who then vote or decide on policy positions.