A closed loop is simply a great way to add LOTS of circulation without sacrificing real estate in the main display. It also helps to reduce the water temperature because the pump is not submerged as with powerheads.
In my system, I have 3- aquaclear 802 PH's, a AC 402 PH, my Mag 9.5 return, and a little giant 3 as a closed loop for extra circulation. The closed loop allows me to achieve movement rates approaching 30X's an hour turnover in my 75 gallon tank. The corals go nuts over it
I would have to sacrifice another couple of PH's and the added heat to otherwise get these kind of flow rates. Not too mention how much NICER it looks without looking at PH's and cords, just a water opening and intake tube, covered with coralline.
To further describe my system's flow rates, ALL 4 AC powerheads are run from a Wavemaster Pro. I really like the circulation and random currents it generates, but it also eats these AC PH's alive. I rip through an impellar ($18 each) about every other month on avg. So, the closed loop is first of a few that will replace this whole PH ordeal. I will either use the SCWD's (People still using these things?) or a Sea-Swirl for randomization of current.
My refugium is a 55 gallon tank that runs about 3/4 full of water (and sand). Flow rates through the sump are on the order of 8X's an hour or so. A bit fast for a refugium, and I have a plating Monti right under the return that shows it. The Monti has a small bit of red slim algae growing here and there where the return pushes out over it. This means nutrients are coming back to the tank itself, and the refugium does not have time to do it's job well enough. It might also be indicative of an undersized skimmer, fwiw.
A Laminar flow is something that is not desirable in a reef setting. Laminar flow simply means a non-changing flow from a single point. A perfect usable definition is a Powerhead. Ocean currents are highly variable and chaotic by nature. This helps to keep detritus from settling because a rock gets hit with water from all sides, not just a single side. it also helps to penetrate a thick shrub of acropora better, washing away the waste toxins. Hopefully you get the idea of the importance of a random flow. Not only is turnover rates important, but the Movement of the water as well.