While a 10" slick isn't necessarily needed, the more rubber you can get under a car, the more "insurance" you have that it will hook, particularly at local tracks where prep may not be as good as it can be. It takes some work and can get expensive as well to properly sort out a suspension so it does hook when there are limits in the rules, like 9" slicks. People try different combinations of converters, gears, cams, etc. Same can apply to suspensions if you want to get it as efficient as possible.