There is always fungus growing under the fabric, attached to the mulch. The bleach solution kills those spores and hopefully disinfects that mulch as well.
Rather than simply a basic ingredient, sodium hypochlorite, outdoor bleach has several ingredients that combined, produce more cleaning/disinfecting results. At least I think it does. Since Outdoor Clorox came into being, I have always used it outdoors on all kinds of hard surfaces but I also spray it on the shower tile in our master bath.
