You wouldn’t leave every piece of paper that lands on your desk piling up there forever. At some point you would throw it out or file it somewhere.
But that’s exactly what most people do with their email – and that’s a good way to get overwhelmed and miss things.
One quick note: This is written for individual users who don’t already have an organization’s email policy or a legal requirement telling them how to handle emails. If your workplace has specific rules about retention or filing, those come first. This is just for your own inbox, on your own time, under your own rules.