You are more likely to demagetize your room key with a cell phone or magnetic closure on a handbag, than with a credit card, but it can happen. I try to keep mine totally separate from any other card and not near my cellphone.
When you check in, they will ask if you want room charging privileges on your room key. Most of the time I say no, but if you say yes, then they will tell you the maximum you are allowed to charge and a hold is placed on that amount. At this point, you can use your room key just like a credit card to buy food, souvenirs, or pay for recreation. I believe they can reset the maximum if you get up to it, but since I prefer to actually use my cc if I want to purchase instead of using the KTTW card, I have not been in that position. At the end of your trip, you will receive a printout of your room folio, listing everything charged to your credit card, including the room nights and anything you charged on your KTTW card. It's a good idea to look it over carefully, as it's easier to fix things while you're still there instead of after you get home.