As most have said here, you can customise a load for your your gun by reloading and gain accuracy, you can change bullet weights and configurations when you want, and you can save money.
Recently a good friend of mine who is a fairly "green hunter and rifle shooter" bought a new .308, he took it to the range and made a nice 3 shot group at 100 yards with factory ammo. When we got talking I tried to explain to him that he could get more out of the rifle with reloads which he was quite hesitant believing. So we went to the range the following weekend, him with factory loads and me with factory and reloads for an experiment. We set up targets at 100 and 200 meters, he shot a nice group again at 100 and when he shot at 200 had about a 6-8 inch spread. I pulled my rifle out .308 as well and shot the same distances with factory ammo and achieved similar results as his. Then i pulled out the reloads I have worked up for my rifle and shot at the same distances again, he was shocked as hell when the 5 shots at 100 were pretty well in the same hole and the 5 at 200 were all touching. Long story short I ended up building a load for his rifle as well.