Rendered art is a gorgeous sight to behold. Your eyes (at least, my eyes) get all gooey and detail-happy and it gives an otherwise nice illustration an extra boost.
Arzach by Jean “Moebius” Giraud, page 33-34. EYEGASM!
In comics, however, this kind of rendering can not only be extremely exhausting for the artist, it can also be exhausting for the reader! Too much rendering all the time can oversaturate our brain’s capacity to notice differences in detail.
Why do we want our readers to notice changes in detail? Well, an artistic upgrade can help put an extra ‘oomf’ to an otherwise OK panel when something important is happening. Let me elaborate.