What’s the value of content previews?

Content previews let you see how your content will look before it’s published.  CMSs have long offered previews, but preview capabilities are becoming more varied as content management is increasingly decoupled from UI design and channel delivery. Preview functionality can introduce unmanaged complexity to content and design development processes.   Discussions about previews can spark stong opinions. Are content previews: Helpful? Unnecessary? A crutch used to avoid fixing existing problems? A source of follow-on problems? All of the above? Many people would answer previews are helpful because they personally like seeing previews.