The assumption being that developers need to test as they develop. And are a more informed user.
(In addition to people always being able to recompile the browser with whatever modifications they want, of course.)
Developer edition is what used to be known as "Aurora", which is in between Beta and Nightly.
"There will also be special unbranded versions of Release and Beta that will have this setting, so that add-on developers can work on their add-ons without having to sign every build."