Maybe I was just not desperate enough when I was younger. I always had my favorite manuals in a pack with me. One military manual and another one that was commercially available. Trying to catch fish with a safety pin as a fishing hook is pure pain in the ass. Seriously. I'm sure some people have done it but I cannot see you bringing in much more than a sunfish or small spot or something. Which will provide you with some food...but nothing like a fish hook of even a smaller size than the safety pin you are using will.
The main reason why they are in a part of every survival kit, including military fishing kits that already have a very good supply of fish hooks and basic lures in them is because clothing gets ripped and torn in the wild, etc. It can be, doesn't mean it always will be. You can make a quick repair with a stout safety pin. The safety pins in military fishing survival kits are the same size and type that are used to fasten stripper clip guides to bandoleers of stripper clipped ammunition. They are heavy duty and they are either back or silver chrome plated. I have seen both types.
Although they suck, in my opinion, as rudimentary fishing hooks, you could catch very small fish with them.
They can be used to repair torn clothing, as pointed out earlier. Until you have the time to use your sewing supplies to repair it properly - or you might not have enough thread for a jacket or coat that really got ripped open, etc.
In other words, they can keep you warmer than you might otherwise be if your outer garment were damaged...
You can make certain small tools out of them - beyond the scope of this conversation.
You can use a safety pin, or a sewing needle, to dig out thorns or other nasty things you might get stuck with in some type of survival situation.
You can use them as a securement for a lanyard. According to one USAF Pilot interview I have on DVD, the USAF learned early on in that conflict that Pilots tended to lose valuable equipment that they had in their Survival Vest. So they started attaching everything to a terminal point inside the pocket with a lanyard using Mil-Spec Accessory Cord (Not ParaCord but available from suppliers like BestGlide and well worth the investment as well.).
You could do the same thing with a couple/few vital pieces of gear and do it easily with a couple/few stout safety pins which could easily be removed later when the piece of equipment is stowed back into the pack.
What I like to use them for most of all, someone else already pointed out, they are great for arranging fish hooks on! Didn't learn that one here, though. Have to thank Bryan Breeden for turning me onto George Jasper's Survival Book, also highly recommended.