The whale feeds on them.

When he desires a meal he opens his great mouth and rushes into the midst of a shoal of medusae; the little things get entangled in thousands among the hairy ends of the whalebone, and when the monster has got a large enough mouthful, he shuts his lower jaw and swallows what his net has caught.
The wisdom as well as the necessity of this arrangement is very plain.
Of course, while dashing through the sea in this fashion, with his mouth agape, the whale must keep his throat closed, else the water would rush down it and choke him.

Shutting his throat then, as he does, the water is obliged to flow out of his mouth as fast as it flows in; it is also spouted up through his blowholes, and this with such violence that many of the little creatures would be swept out along with it but for the hairy-ended whalebone which lets the sea-water out, but keeps the medusae in.
Well, let us return to our "cutting in".

After the upper jaw came the lower jaw and throat, with the tongue.

This last was an enormous mass of fat, about as large as an ox, and it weighed fifteen hundred or two thousand pounds.