why iluminate retical ?

What price range scopes are you talking about though? Not all illuminated reticles are of the same quality. I can't speak for the $1K+ scopes but comparing sub $500 to $500-1K scopes can be night and day. I don't think it's necessary to have illumination but it helps if you're targeting something with a dark backdrop like the woods even on a sunny day. It also helps to have clear optics and a fine reticle for airgun use. I've seen some illuminated reticle that's just a blur once you turn it on. So yeah, not all illuminate reticles are the same and until you use one on a better quality scope, it might not suit your eyes. Plus on higher end scopes, the line of scopes should offer illumination as an option to save you some coin if you don't plan on using it anyways.

I live on a 60 acre spread in West Tennessee and there are a number of pests and some small game animals that are nocturnal. In the last dozen years the armadillo has taken up residence and become quite common here. Just one of these critters can ruin a lawn in a couple of nights. If you raise chickens or guinea fowl the rats, coons and skunks will come looking for eggs, and the coyotes will come looking for the chickens and guinea fowl. Last year, my father, who lives on a 1 acre plot in northern New England, was overrun with skunks. All of these challenges were handled in the moonlight using an illuminated reticle. I've also used my illuminated reticle during the day when I was in the sunlight and the quarry was heavily shaded. Black mil-dots are difficult to see against a black background. Not everyone will need an illuminated reticle but it sure is nice to have when you do. 

I was using mine earlier this evening, and forgot to turn it off...thanks for the reminder