I agree it catches way too many people off guard, they should just disable it and give a progress bar instead. To explain it: That's the actual output of all the maps combined (same thing as running a "complete map). What it shows is a full render of the object, then based on your settings it saves out pieces of that…
What should it show you when you render a diffuse, lighting, AO, and normal map? I think showing the complete map makes the most sense when you start rendering more than one map, and if it was inconsistent it'd be confusing.