Fiberglass fabrics are hugely common in the composites marketplace due to there much lower costs over other fabrics. Use fiberglass wherever you don't need the super strength of carbon fiber. Fiberglass is white in colour, however turns clear once resin is applied. This is a useful feature as it can be used as a clear top layer over other fabrics including kevlar, which in the case of kevlar gives the advantage that the fiberglass can be sanded smooth.
Considering our customers are generally looking to make carbon parts, we have found the biggest attractions for using fiberglass for our customers usually comes in mold making.