See Infiltrator Weird.

Another, simpler combat rule that hasn't been altered by a keyword yet. It might be too similar to vigilance, though of course it has some key differences.

Am I missing something here? I don't see any nonregular scenario where this is relevant. Vigilance obviously allows a creature to block the turn after it has attacked. However, if a creature has attacked, it becomes tapped, and then before its next usual chance to attack, it is untapped again, so the keyword just doesn't do anything there.

Obviously it can be used with some activate granting abilities that require a creature to tap but those are the "nonregular" scenarios I referred to earlier.

Hmmm. Well, on a non-0vanilla it lets you use the ability then attack. Of course, this example doesn't have such an ability.

And it does work really REALLY well with a second combat phase, if you can gain one of those.
But I agree, it doesn't really seem widely-useful enough to be keyworded. Though I can certainly see some cards having it.

No, you're right. It is quite narrow. The main use, outside of being able to use a ability before attacking, would be negating effects like Thalia, Heretic Cathar and other tappers by being able to attack through them.

But it needs to also have haste, otherwise entering tapped doesn't affect whether it can attack

That's correct.