Why is detail important in drawing?

Because you need to see what the persons feeling and be able to look at its facial expressions.

It depends on what type of drawing you are doing... Usually in cartoon's detail isn't really necessary, but in fine art or portraits it is essential because you need others to be able to know what it is you have drawn. Say if you are drawing a circular table and you're lacking shadow, texture, light, e.c.t It will basically only look like a circle with sticks pointing out of it, or if you're drawing a face without any detail it will basically be a lifeless plain oval, with eyes, ears and e.c.t stuck on it. Details are what makes your drawing life like, real and awestrucking.