I couldn't have put it more simple than Emma! +1
First time I saw the term 'mdeploy' I also had a misconception ... thought I might be able to build a moodle to a point and then share that to whomever had the download link. You might think of it as from the perspective of the 'moodle store' where one can find plugins (extra mods/blocks/themes/code customizations,etc.) - deploys from moodle code/plugins server.
Still though ... mdeploy is the way to update ... IMHO ... 3.3.x has much better checking on the compatibility of what you are about to install or update/upgrade. Doing it manually or drag and drop still has the potential for human error.
As is good practice, best to do a minimal site backup of code and DB before pulling the trigger on any update/upgrade ... be that plugin or site.
For that one could easily create a bash shell script(s) and those could be accessed via the web if one built a local_snapshot plugin.
Realize that's more than you wanted, but thought I'd throw that out there! ;)
'spriit of sharing', Ken