Let's say you've built a custom form for your Drupal 8 site. It contains various elements for input (name, email address, a message, that kind of thing), and you want to send the submitted values in an email to someone (perhaps a site admin). That's a pretty common thing to need to do.
This could be done with Drupal's core contact forms, webforms, or similar -- but there are cases when a bespoke form is needed, for example, to allow some special business logic to be applied to its input or the form presentation. The drawback of a custom form...