The external unit is mainly for controlling the levels of your effects going through the loop. As you turn up
your effects are going to get to hot running through the loop and will be a bit washed out.
Also if you play out like I do the external loops acts as another master vollume so you can get your sweet spot on the amp
and then turn it down through the external unit. I played out once without it one time when I had a bad cable in my board
and I didn't have time to find it since I had just hit the stage and the sounds was not as good. It really warms up the amp
when running effects. I ended up getting the Glaswerks Matchbox and will be upgrading later to his  reverb unit.

Here is mine in action with an Eventide Time Factor run through a Suhr Mini Mix 2 into the Glaswerks in parallel.