While most of the answers you received below are correct in what they are saying, none of them really explain why it's in your best interest to sign this agreement and be represented by a Buyer's Agent.
An agent is either going to represent the seller, the buyer, or sometimes neither party. If the real estate agent represents the seller, then their allegiance is legally with the seller. Not a very good scenario for the buyer. In that scenario, any of your information may be shared with the seller. This doesn't happen often, but legally if the agent is not working for you, then most of the time they're working for the seller.
On the other hand, if you sign the buyer agency agreement, they will then represent you, and any information they have or can gather about the seller, will be shared with you, and all of your information will never be shared with the seller. The agent negotiates on your behalf, and represents you the best way they can. They should be looking for all the pros and cons about a home and pointing them out to you as well. Since the agent views homes all the time, and presumably, you only buy a home once every 5 - 10 years, they will spot defects that you won't.
I hope you found this helpful. If I can answer any further question for you, please feel free to call or email me. I'm located in Reston as well.
Keller Williams Realty
#1 Individual Agent in the Reston KW Office in 2010, 2011, & 2012
(703) 599-4755 - cell