Alright, lets say I have an account named 'Test'. This is an account that I've spent forever on. It has DLC, high prestiges, 50 in Halo, etc.
Then lets say someone is selling the Gamertag "Shred". It has no stats, but I want it because the Gamertag is rare and is cool.
I buy the account, but want it on the 'Test' account because of all the DLC, prestiges, etc. I have. I then go on the account 'Shred' and change the gamertag to something random.
I then hurry on my main account, 'Test', and switch the gamertag to "Shred".
Someone with a Turboer can swap/take the account before "Test" can change his gamertag to 'Shred'.