Why are credit card details necessary?

The hotel requires your credit card details to guarantee your reservation ONLY (except non refundable reservations like Early Bird, Hot Deal or Advance purchase).
Your card will NOT be charged until your arrival.
At this stage you may be asked to pay by cash or with an alternative credit card accepted by the hotel, this is your choice.
Usually the payment is on your check-out. Some hotels might require you to produce a method of payment upon arrival - whether it is cash, bankers cheque or credit card. 

Only if you fail to cancel your reservation in time (cancellation conditions are on Reservation Form, Reservation Summary and your Voucher) and do not arrive at the hotel, the hotel may charge your credit card with a cancellation fee - usualy equal to the cost of the first night´s accommodation (written on confirmation Voucher you recieve). 

Most hotels accept all major credit cards, however, there are exceptions.
You can see on the page of each hotel a list of the credit cards that are accepted by the hotel.

You can also find further explanation on the reservation form.