If you are coming from a previous generation virtual world or an environment that does not make use of PBR (physically based rendering) materials, it can be just one more confusing thing to wrap your head around as you learn how to use Unity to make content in SineSpace. Make no mistake, learning to use substances with SineSpace is worth your time.
Your regions are limited by their filesize in SineSpace. The biggest contributor to the size of a region is your texture bitmaps. Substances are fairly small files that get rendered when people visit your region. Therefore your region is smaller and downloads more quickly. Smaller regions also means more room for furniture and decor (you can never have enough room for that).