I'm not certain exactly what xchange's policy on that is. But because it's a financial transaction they may be using your birthdate as an added verification you are the purchaser, if at a later stage you request support or need to re-download the software.
I dont imagine it's anything more than that.
Some banks here ask for things like mother's maiden name, or similar. It's just an additional means to verify you're the person making the transaction if you need to follow it up.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"