Ok if I can buy old guns without a C&R then why get one ?
Yes, you can still buy "old guns" without a C&R. With a C&R, you'll send copies to the big dealers and they will send you flyers, emails and you'll find a ton of firearms you didn't know you really needed. You'll remodel your house to have storage and display space for your growing collection.
Every time the UPS truck (better known as the BBT: Big Brown Truck) rolls down the street, you'll think "cool, what did I order again?"
The advantage is when you go to a show, you see something that the C&R, give them a copy of your license and $ and walk out. The only phone call is to place an order (once your C&R is on file with the seller), not to do the background check.
Yes, you have to keep a bound book. Yes, the BATFE can ask for an inspection, just play by the rules. Remember that you are not a dealer, the C&R is for your personal collection, not selling. With these small responsibilities, you have a greater freedom to purchase C&Rs in different states, etc.
I'm surprised nobody started wailing about the record keeping (bound book) responsibility yet. Usually, when someone asks about C&R, there are some nay-sayers. IMO, if the record keeping ever became a tool for confiscation, we have much bigger problems to deal with!
Just be ready for the stress on your wallet with a C&R. It's a lot of temptation!