but this is same as Update in CM ... and depending on the type of update the author did existing files could need to be removed (then clean install is needed). Or if a update is mandatory (when no clean install allowed, as the archive with the update contains only the needed files of the update , and is not a full package containing the original files which remained the same).
If the author doesn't specify , one doesn't know for sure.... (unless you first check and compare the content of the archive provided with the actual files in the previous version, to figure out what is the best course of action).
I hope you see that not all of us want to do that last part, and it's easier for the author of the update just to always pass that info along with any update.
EDIT : in response to Mascot's "always do clean install ... "
I also used to think that way , but now I do not completely agree anymore....
as you could end up w a none-working partial mod if the update archive is not a full package (if files that were already in prev. version are omitted in the update archive ).
And if a clean install is not needed, I prefer not doing so , as it means more extra work (creating new custom previews, re-adding custom skins, ...).