If you read the readings here, and the number explanations on the first page here, you'll soon see the significance. :)
Basically, the Birthday reveals several things:
The day of birth is very revealing about your personality in general, the kind of people you will just naturally attact to you.
The life lesson #, which is your b-day added up, is what you came here to learn, so again, what you are exposed to so you can get in the experiences you wished to have.
The b-day also reveals challenges and pinnacles, more aspects to the personality that need to be balanced or taken advantage of in order to succeed.