A lot of them have a pet cause they want to force into the national spotlight. “Running for president” and (attempting to) force the major candidates to address the issue as part of their campaigns is about the biggest spotlight you’re ever going to get. Most that announce a candidacy for president have an agenda completely unrelated to becoming president.
Think of the “The rent’s too damn high” guy. He had no expectations of winning anything, he just wanted to make sure everyone knew that the rent is too damn high.