Lighter on memory, preferred on Sun 3 by many, compared to X Windows. Ported to PC, Atari ST, Macintosh. Here are historical notes and even sources:
The MGR Window System
MGR, sometimes said to be short for “ManaGeR”, sometimes short for “Munger”, is a simple network transparent window system. It was originally developed for the Sun 3 series of workstations by Stephen Uhler and colleagues beginning in 1984 while at Bellcore
Many people at Lysator prefered MGR instead of using Sun’s own SunView (nee SunTools) and, later, many still preferred MGR instead of running the X Window System. X, even on the high-end 3/80 workstations we later had, was not a pleasant experience, probably mostly because of memory starvation. It wasn’t until we had SPARC CPUs that most users started abandoning MGR.