Creatures and the In-human lifestages (not the sims 3's Occults, a word which I really dislike) were added to EPs because of the themes they brought with them:

Basegame - Aliens and Ghosts (Relating to Genetics and Life-spans of sims respectively.[[Both new in the series]])
UNI - Zombies (Brain-dead, working without sleep, etc.)
Nightlife - Vampires. (Obvious.)
OFB - Servos (Crafting was a big theme.)
PETS - Werewolf (Again, more obvious.)
Seasons - Plantsims (One with Nature.)
Bon Voyage - Bigfoot. (Not really a good one there, EA.)
FreeTime - Genie (Again, not exactly amazing. Zombies were also re-added but they were exactly the same.)
Apartment Life - Witches (It's the last EP give them something they want slash that witch upstairs is banging on the floor again gheez call the landlord)

I didn't get any of that themeatic material from the few Sims 3 expansions I purchased.. even more with the Sims 3 "Throw a bunch of creatures into one EP pack" of SPN. I'd prefer Sims 4 to take a similar route with Creatures (as I do /adore/ them in Sims 2) but I'm more in want of Expansions which (thematically aka through common themes and genres) expand upon the game without just adding lots of New Stuff. I want to see my sims lives enriched!