Standard laundry detergents are extremely harsh, and often contain heavy surfactants, optical brighteners, perfumes, and various other additives. Each of these will become embedded in the fabric of your garment with every wash cycle.
Normally, this wouldn't be a problem - these additives have a specific function when treating standard laundry items. In technical, or waterproof clothing, however, they instead act as wetting agents by masking the DWR treatment your garment possesses. In short, not only will your garment fail to perform as it should, any subsequent reproofing will not be as effective until the additives and residues are removed.
Granger's Performance Wash and Down Wash have been specifically formulated to effectively remove dirt and unwanted residues from performance textiles - helping your garments maintain their existing DWR finish. As well as this, Granger's washes also provide a clean, residue-free base for effective reproofing.