So we can send you singing telegrams, of course! All jokes aside, securities regulatory authorities and the IRS require all brokerage firms (including ShareBuilder) to jot down your home address, even if you don't receive mail at this location. An account cannot be opened without this information. Also, errors in these fields may prevent your account from being approved for trading, which isn't what we want to happen, either.
If you choose to receive mail at an alternate mailing address, you can list this address at any time.