They've already given me a card, which I pay off every month. Why are they interested in how much I make?
The main reason for an income check, beyond marketing purposes would be a credit limit increase. If you didn't apply for one, then it is most likely that they just want to keep their information on you up to date in order to mine better marketing data from you.
That is to say, if your income has changed, and your spending patterns change, they can use this data to get a better picture for their demographic marketing.