When the first act opens, Bernardo appears to relieve Francisco from duty, but it is so dark they both fearfully call out "who's there?" until they realize it's just guardsmen. We find out that the guardsmen have been spotting the ghost of King Hamlet.
It means, for example if I say, "I go to the park every now and then", then it isn't a common event, but happens often enough not to be classified as a rarity.
He wished people would attend to details. :)
To show who is talking. Who specifically.
