From Day 1 - Apple 2s - Macintosh - Quadra - were pro office machines.
Macintosh a pro machine? Get a clue. Macintosh was Apple's first consumer computer. Apple II and Apple Lisa were higher priced more prosumer oriented machines while the Macintosh project from start to finish was aimed at home users who wouldn't want to pay a lot of money for a personal computer. It used a much slower CPU compared to other Apple machines at the time, and one that was much much slower compared to intel offerings.
Quadra (I owned one) was a fast machine, but still a prosumer class machine due to motorola chips. For Apple, it was their high end professional machine, but if you looked at other offerings from all brands, if you wanted actual computing power you'd have to go SGI at that time.
G3's towers were faster than intel at the time. They won't discontinue the towers - pointless. Apple have always pushed the boundaries of technology - what they going to use to design the next gen of iphone? An i7 or a 12 core number cruncher.... or how about a windows machine? I expect they did not updated the Mac Pros last year as the last intel update was not worth the extra horsepower and/or they were waiting on the very delayed Sandy Bridge E5
G3's were slightly faster than Pentium 3's. They weren't faster than Xeon's. Intel offered workstation class processors long before Apple switched to Intel. Not that it mattered. When Apple released G3's, Windows was years ahead of Mac OS so any speed increase coming from the processor was lost due to outdated system architecture.
And again, at that point in time, SG offered the fastest workstation class video oriented computers.
They were faster for graphics applications which was the main reason Pros used them. PC's were faster in the games area. I do agree that was in part due to the OS being far mor efficient than windows
That was only at the early times. For a small while, Photoshop worked faster on Mac 68k processors than it did on Intel offerings on Win machines. That didn't really last long and that couple percentage of speed difference was NOT the reason why people used macs. Macs were easy to use, and desktop publishing folk are not computer experts. So they chose the mac due to ease of use and less everyday issues.
Basically for a very long time, SGI was THE computer to use if your work demanded a lot of CPU time. Almost all post production work was done on SGI's until the early 2000's when Intel offerings started to be "fast enough" for most of the work done so some post production did switch to Intel's due to cheaper price than SGI workstations. Then when Apple released G4 and then G5, they did attract some people, mostly due to the price of their software offerings (Final Cut was 1/10th the price of Avid, Apple Shake was 1/5th the price of Shake, Apple Color 1/10th the price of Color) etc. The computers were still slower than Xeon workstations, but they were fast enough for most work and the software price difference was considerable.
I would say that Apple never sold professional level machines until Mac Pro, which was truly the first niche product Apple came up with. But some pro folk did use Apple because Apple's prosumer machines were "fast enough" for their work. Desktop publishing is the area where Apple did shine, because of less need for CPU power compared to video, 3D etc.