I'm a Roll20 Pro subscriber, and I use the Mods system to automate the monsters in my 13th Age Glorantha game (ScriptCards), and to make setting conditions quickly and consistently (TokenMod).
Otherwise the Transmogrifier is the Pro feature I use the most, because I can set up Library games for monsters, maps, etc., and keep my active games small & tidy. This used to be more important before they implemented more dynamic loading.
For example, I have a 13G Library game with a complete bestiary ready to go.