Why does Justified exist?

Question

I know you can use Beat Up in doubles to boost your attack, but in singles no one will think of hitting a fighting type with dark moves.

Comments

Switch ins maybe?

Answer 1

Switch-ins

Justified "exists" so that when you predict a Dark type move to be used on your Pokemon you can switch out to your Justified Fighting type and gain an attack boost.

Example:
Your Mew is up against a Tyranitar that you know has Crunch since it used it before.
You predict it will use that move (because Mew is weak to Dark) and you switch to your Lucario that has the ability Justified and gain an Attack boost.

Answer 2

One word: switching

In competitive battling, everyone switches their Pokémon around, Justified is a great ability that takes advantage of this, as the only Pokémon with Justified are all Fighting types, so most of the time they can safely switch in and gain an Attack boost. It also deters opponents from hitting Pokémon with this ability, unless it is Keldeo which almost always runs special sets. Those are ways it can be useful in single battles, and as you said it can bring you all the way up to +6 in double battles with Beat Up, and is even possible in single battles with prediction.