Sometimes, airlines overbook, which means they've added more people to the flight than there are seats. Most people don't even consider giving up their seat for someone else, but doing so certainly comes with benefits. Most airlines offer significant cash compensation and will even help you book a new flight for free. If you have to be somewhere at a particular time, this might not be something you'd want to do.
If your travel plans are flexible, there aren't really any downsides to giving up your seat for some extra cash or travel credits that can be used towards another epic trip.