Why do you think this "stranger" appeared in the town of Hogsmead with a dragon's egg?

This is my time to shine. HP questions.

The stranger appeared in Hogsmeade to order Hagrid, the gamekeeper of Hogwarts, enough drinks to get him drunk enough to tell them how to get past Fluffy, a three headed dog, and get to the sorcerer's stone. They appeared with the egg to get Hagrid willing to have some drinks with them.

Explanation:

All explained in the end of the book.