It’s tempting to put this in the pit as a rant, but there is a GQ also.
I’ve just received a new credit card and the card company has printed the number down the middle of the signature strip. This makes it difficult to sign as the printing deflects the pen. It will also make it more difficult for the merchant to check the signature.
There must be a loss-prevention reason for the cc company to do this, though. What is it?
There are two primary reasons that I know of:
To prevent people from scraping off the top layer of paper and then putting on a new signature, since the numbers won’t be there any more.
After your credit card number is a three digit Security Number which is now being used by some internet merchants for authentication. (That was the original purpose of asking for the expiration date, so I dunno why they’ve added yet another number.)
I believe the idea is that being able to provide that number proves that you are in physical possession of the original card, as opposed to having intercepted the details or copied the information in the magnetic strip.
The printed numbers would be removed if someone tried to erase the original signiature. Also, if the embossed numbers are altered (which is fairly easy to do), the printed ones on the signiature strip are harder to change. Further, merchants who need to enter the numbers manually find that the printed numbers are often easier to read than the embossed ones. The printed numbers also include an extra three digits as an additional security measure for use in phone-in or online credit card orders.
The expiration date is embossed on the front and is captured along with the imprint of the card number. To prevent someone from getting hold of an imprinted receipt and having that information available, the 3 digit security code was implemented. since it is on the back of the card and not embossed, nor contained in the magnetic stripe, this number will never appear anywhere on a transaction receipt, whether manually enter and imprinted, or scanned electronically.
I don’t find it to be that much of a security measure. I just ordered a new Visa card & they ask you to call them when you get it. So I called, they asked a few questions, including what those three digits were, which were the same as the old card. So I asked them how they new the old card from the new one cause all the numbers are the same on both cards & he said their computer knew.
The OP could have been clearer - I meant to ask specifically why they are now printing the number on the signature strip as opposed to anywhere else on the back. I understand and agree with the reasons for printing a number (plus the extra security ID) at all. For the reason it is placed on the signature strip, friedo probably has it right:
Credit card fraud analyst here…
To confirm what others have said, the number is a security measure. It proves possession of the card.
Think about it this way. If your card is stolen, at some point you’ll notice it’s gone. If the account number/expiration date are stolen, you would be none the wiser. With the increase in online fraud transactions, the Signature Panel Code proves who placed the order has the card, not just the information.
It also prevents counterfeiting of cards. I can’t really say how without saying how counterfeits are made and how we catch them. Sorry, can’t give away the store. 
Q.E.D pointed out the number is only on the card, which is true. We don’t have the number on file at the bank. It will not be on a statement or a receipt. We have to request it from the cardmember.