The reason is you can make food that gives you great buffs.  You can get foold form vendors but the buffs are not as high and dont last as long.  If you raid haveing a 23 spell dmg buff is great.  As well as haveing 30 stam and 20 spirt in a BG.  It is worth the time to lvl Cooking up IMO.  Make your choice  but taking 5 mins out of every few hrs of play time to lvl cooking is not that much of an effort for the prize at the end.