Was Pokemon Blue necessary when we had Pokemon Red? Was Pokemon Gold necessary when we had Pokemon SIlver? Was Pokemon Ruby necessary wehn we had Sapphire? Was Pokemon Diamond necessary when we had Pokemon Pearl? Anyway, now that is over with, here are thye differences between Sapphire and Emerald:
The TrainerEyes feature has been changed to the Match Call. Sure, you may think its just a name change, but now you get rung by the registered trainers!
Sure its only minor, but the Pokemon Sprites are now animated! (Like they were in Crystal)
You can get both Kyogre and Groudon (and they are both at level 70!)
Storyline is longer and (in myopinion) more epic!
You can get Rayquaza before beating the elite four. (Yes, he is still at level 70...)
You fight Team Aqua
and Team Magma!
Rayquaza is the main Pokemon.
New Gym Leader.
New Champion.
You can still fight Steven (and he has quite high Pokemon too. Shame you can only fight him once...)
Some new Johto Pokemon!
If you complete the Hoenn dex, you can get one Johto Starter!
The Safari Zone is larger once certain reuirements are met
You get the national dex once you beat the Elite Four. That is the only requirement...
Battle Frontier (Battle Tower and more facilities.)
More Base stuff.
Wireless adapter compatible
Minigames (same as FR/LG)
Sows route names (not sure if it does this in Ruby. Even so, they have pretty cool graphics.)
Protoganists have new outfits.
You can catch a limited Sudowoodo!
You can get both fossils!
New places: Mirage
Tower (notm island) and Underground Pass (I cant remember what it's called.

You can find the fossil you didn't choose and Ditto here! :D