You may find the
Cloud Service FAQ helpful.
Cloud Service gives you offline access for your players (if they have at least a Player account in Realm Works). If you don't have cloud service, you can only use the player view to show things to your players.
It also lets you merge realms. If you don't have cloud service, you can duplicate your realms as well as any purchased content realms but cannot combine realms at all. (So a given campaign realm could only have one source realm.) If you have cloud service, you can merge as many realms into your other realms as you want, as well as duplicating them.
Last, but not least, it's a way for you to backup your realms and bring them to other machines. :)
I may have missed things, but those are some of the big ones.
Hope this helps.