The Sims 3 Store predates Origin by a couple years, and Origin has never been set up to handle Sims3Packs. So rather than spend time and resources adding that functionality to Origin, the people in charge kept the store and moved purchases to Origin after a few years. This has the added advantage of allowing people who don't play through Origin to still buy worlds and install them like everyone else.
If you buy a physical copy of a store world, you'll get a box that has a code in it and may or may not also have a disc. But even with a disc, you'd still need to register the world on the store site before installing it; I don't think there's a way to make the world install properly without that.
If you buy a world through Origin, you can definitely send the product code to someone else. It should appear in their store purchase history after they register it.
Store content is encrypted and can't be used by anyone other than the person whose account downloaded it. If the product code for the current installation doesn't match the product code of the account, the content won't work.
