Additives are not necessary but can be helpful sometimes or give a result that other things/flavors cannot. For example, Koolada adds a cooling sensation to your juice. It's similar to the way menthol is (regarding the cooling sensation) but there is no flavor. I really like it. It makes mint and certain desserts pop. Some people also like adding this to fruit. (I personally don't like mint+fruit...ick)
Menthol is a nice additive if you like menthol. Many people don't, myself included. It reminds me of cough drops, which I hate. Very rarely do I find a mentholated juice I like but it can be tasty if utilized properly.
Some people like E.M. as a sweetener but it does effect the flavor of whatever you're adding it too. Some people like that flavor, some people hate it. On it's own, to me, it tastes like cotton candy... which is how many vendors make cotton candy flavored juice.
Smooth, Vape Wizard, and all that stuff... I am not convinced that they do much of anything. It could just be that I don't know how to use them but I have never noticed a real/dramatic difference when I use them.
Bottom line is that in general, they are not necessary and if you're just starting out, I would avoid them until you know what you're doing. (no criticism with that statement... just trying to save you some frustration. As indicated above, additives can ruin a juice.)