Thumb trailing is just one of many muting techniques, and it’s not always useful (that’s why there are other techniques as well). Actually, muting is only a by-product of thumb trailing, and only the string above the one you’re playing is muted.

Thumb trailing, however, has a practical effect on the ergonomics of the picking hand. A small angle between thumb (which supports the picking hand) and picking finger (or index and middle fingers when alternate picking) results in increased picking finger energy and greatly facilitates sixteenth playing at 120bpm, for instance.

You can easily verify the thumb trailing effect even without a bass. Move your index and middle fingers quickly back and forth similar to alternate picking. Now increase the angle between your thumb and the moving fingers. You’ll slow down noticeably and you need more energy.