I've always figured it was so they could tailor their comments/recommendations. If they know you've been there before they won't need to explain as much.
Also, if you've been before, and stayed on property, all kinds of info (like kids' ages) are in the computer already. So, they know how old your kids are, and what ticket they need, etc.
I don't know, but I go at least once a year, sometimes more, and have never gotten a special discount offer. My neighbor went once and got a postcard with a $49 offer to stay at one of the values. My friend called to book her family's first trip and it was more than she expected. She told the castmember they had never been and they gave her an unadvertised discount. I called after she told me about it and couldn't get any discount.
I think Disney marketing is trying to get a feeling for how many of their guests are first-timers as opposed to repeats. Obviously they want both types of guests, but I think they really want to see what type of marketing brings in new folks and what marketing schemes bring repeat guests back.
I've always figured it was so they could tailor their comments/recommendations. If they know you've been there before they won't need to explain as much.