The section header is particularly true if you’re cold-emailing the employer. You need to give them a reason to read your message. At the very least, make it clear which position you’re applying for. If you can personalize the subject line, that’s even better.
Be clever, but not too clever. Show excitement, but don’t appear disingenuous. The person you’re emailing likely gets emails like yours frequently, so they’ve seen it all. If you stray too far from the norm, they may assume you’re unprofessional. But if your subject line is bland and uninteresting, it may get skipped. It’s a balancing act that gets easier with experience.