Sir Swindle:
Sure but the same can be said about Adult. With the added feature that they already have confirmed they are an Adult.
That confirmation being very legally important, because in most places, deliberately exposing minors to explicit 18-rated sexual content is a crime. An age check won't
stop sufficiently determined idiot minors completely - there's a whole section in the ban list for that - but the observable fact of trying is very important so that people on the receiving end of said foolishness/the site's admin don't wind up convicted of the kind of thing that gets you on the Sex Offenders' Register, children in your household taken away, etc. etc.
When running Mature or lower myself, I use Content Guidelines based on the BBFC's rating system, modified per game specifics, so as soon as anyone looks at the intro they know what they're in for. i.e. nothing minors or anyone else can see in a game is what they can't see in a cinema, and if anyone is worried about specifics in Mature games, there's warnings there. No need to hide anything, legally or otherwise, though of course you can if you prefer.
[squints] Yes? I think the question is answered!