Let the feast begin

[Sheldon Dinkleberg]

So...Is there any use for these guys or are they completely useless..?

[Gan and Tiff]

A nifty little berserk reference? No clue. Maybe they're involved in some secret we all overlooked.

[Player117]

They just give you subtle references especially to some characters of DkS2, particularly Alva the Wayfarer and maybe Vendrick or Aldia.

They also give references to characters from DkS3, probably Irina of Carim and Karla.

"Fear not the Dark, my friend. And let the feast begin", like Gandler said here, probably reference the Eclipse Incident from Berserk.

[RdRStar]

Flavor

[淵 Nero Alice]

its their nature to eat if you read their items and head peicem they are also refering to Midir

[Sumppi]

Those tiny buggers are real opportunists. Even though they usually run away from you sometimes they try their luck and try attacking you when you are fighting something else and not paying attention.

[超高校級のEther]

They are tricking you.
I remembered it mentioned Ciris, saying her devoured by the abyss.
Also I guess 'the feast' means to feed on the dark souls, that's perhaps leads to Gale's tragedy.

Just shove a sunlight spear to their faces and send them to hell.

[Rinera]

Well, let's see into it further! Locusts are known to be catastrophic if they move in big numbers. Crops can go bye-bye if the swarm arrives. Perphaps that's why there's a huge swamp where most of them are. They already ruined the place by eating it.

They are supposedly trying to lure people to the Dark, which by their own logic and the item description, is how things should be.

"Did you not arise from the Abyss, and did you not resist drowning in the Age of Fire?"

If we go by the obvious answer, it's most likely that the Locusts had a point to make in order to "lure the people to the Dark", but their own sanity driven by hunger mixes up and they mention a feast instead of the point, along with ruining their own environment.