Transsexuals who have "completed their transition" need to be recognized as their new gender legally. Such recognition is not saying that such people have now gone from male to female (or vice versa), but deeming(1) them to be the equivalent of their current gender legally.
This includes being able to have your birth certificate and other legal documents changed to show your new gender under your new name. Note that I refer to "completing transition" rather than reassignment surgery, because many transgendered people don't get to have surgery, for any number of reasons (least of which is not wanting to).
Such recognition can be important, especially if you are post-operative. If all you seek to do is function in society as the appropriate gender role, what advantage is there in denying recognition of this?
Currently, many States in Australia have legal recognition of post operative transsexuals: Western Australia, South Australia, Victoria, New South Wales. In 2003 I sent for and received my own recognition certificate. This was important for me for two reasons: a form of recognition that I had at last finished transition; and it changed my birth certificate to "F" so that if needed, I could present it when providing proof of Identification.
Small mercies perhaps, but important all the same.
FootNotes
(1) Deeming is a process long used by legal and other authorities. My own experience with deeming was when I worked for the Department of Social Security. If a pensioner had a bank account with an interest rate below a certain rate, it was "deemed" to be that rate for the means test. The government doesn't miss a trick does it?