I use the strap when i have a small fixed blade in a sheath on my riggers belt, i put the sheath behind the strap keeping it partialy in the butt pocket, that way when you wear a t-shirt or other garment over your pants no one will see the knife.
Other uses include cliping a carabiner with a set of keys on it, although i prefer to have them on a grimlock on my right side belt loop.
The strap is also handy for putting newspapers behind since the butt pocket is deep enought to house one.