Commerce 2: Is it 'really' necessary to setup product attributes before adding a simple product?
If the products you are adding do not require different variations, there is no need to add attributes. Product attributes are meant to define unique properties of a variation of the same product. 

An example would be a t-shirt where the color and the size are two different attributes. The shirt has the same print and model (eg. long sleeves), but any given combination of size and color can exist, each having a different price, photo, etc. and its own SKU.

The documentation lists the creation of attributes before the creation of products, so you can choose these attributes once you start adding products.