Why would startups need to do this? AFAIK, at least for small US companies, they can just hire anyone they want. They are not obliged to post the jobs or accept

Why would startups need to do this? AFAIK, at least for small US companies, they can just hire anyone they want. They are not obliged to post the jobs or accept general resumes -- if they want to hire an insider, they just hire him, and no one outside would even be aware that there was a job offering. I have heard about larger companies / government agencies having these kinds of interviews due to internal procedures, but hopefully most startups have more sane setup.