The CSC field is required to process credit card transactions securely.
The CSC code is a fraud prevention tool. Because the CSC appears only on the physical credit card, entering the code provides some assurance that the actual card is in the possession of the buyer, as opposed to purchaser simply knowing the card number and expiration date. By asking your constituents to enter this field in your forms, you are helping to protect your constituents from online credit card fraud.
The only exception would be if you are using recurring payments: see An Important disclaimer regarding CVV codes and recurring payments for further details.