Is margarine necessary for my brownies?
Hello fellow cooker :) ,
by description 


Butter:
Most frequently made from cow's milk, butter can also be manufactured from the milk of other mammals, including sheep, goats, buffalo, and yaks. Salt (such as dairy salt), flavorings (such as garlic) and preservatives are sometimes added to butter. Rendering butter, removing the water and milk solids, produces clarified butter or ghee, which is almost entirely butterfat. wiki
Margarine:
Whereas butter is made from the butterfat of milk, modern margarine is made mainly of refined vegetable oil and water, and may also contain milk. In some places in the United States, it is colloquially referred to as oleo, short for oleomargarine.2 In Britain it can be referred to colloquially as marge. wiki


So with margarine, brownies will be "lighter" and not so rich as with butter because it has not the same "fat" as butter.
That's also from personal experience as i make brownies with margarine.

Baking with butter has two major advantages over other fats: 
1. Flavour. Shortenings are flavorless, but butter has a smoother and more desirable flavour, adding more taste to baking and pastries. 
2. Melting qualities. Butter melts in the mouth. Shortenings or lards do not. After eating pastries or icings made with shortening, you might find and unpleasant film or coating in your mouth. searched

EDIT: i really forgot to answer the primary question "Is margarine necessary for my brownies?"... the Answer is yes and no. it's all about what you like the most! Except this consider that the cooking art is all about a try/failure duo and a little bit of "alchemy" (i like this terminology :D )