Like Evildog said, I too spend alot of time at the range, usually with my wife or one if not all of my kids (target shooting is our family "sport") and I compete a few times a month, so having bulked ammo on hand for us is more of a necessity than anything. Even when I go to the range alone, it is a common session for me to blow off 2-300 rounds in a few hours. Now, with the family, we will easily go thru 600-700 rounds.
Another reason is the price, as others stated. Although alot of my supply is handloaded, the rest of it is store bought, and with the prices going up by the week, ya never know when the day will come where a box of ammo will cost as much as the gun itself.
I also like to keep a certain amount of extra on hand for each weapon....just in case some politician decides to start something with another country or group and the crap hits the fan here on American soil or that same politician succeeds in banning firearms and or ammo sales.