What secret does Bilbo share with the dwarves? Why?

Bilbo share the secret of his ablity to disappear.

“I am going to disappear,” he said. “I shall draw the spiders off, if I can; and you must keep together and make in the opposite direction. To the left there, that is more or less the way towards the place where we last saw the elffires.”
It was difficult to get them to understand, what with their dizzy heads, and the shouts, and the whacking of sticks and the throwing of stones; but at last Bilbo felt he could delay no longer—the spiders were drawing their circle
ever closer. He suddenly slipped on his ring, and to the great astonishment of the dwarves he vanished.