Not necessary but doesn't hurt and "probably" promotes a bit more growth but that depends also on your water parameters, calcium/alk/mag and lighting.
I feed cylop-eeze on occasion and spot feed my lps every once in awhile but everything does just fine without it as well.
I guess my take on coral food is -- it's beneficial but hard to measure and overall not needed. Depends if you think the expense is worth what you get out of it. I also think my coral gets a bit of food from when I feed my fish or the fish poo as well.