How can this be a legitimate question-- without spawning a laundry list of similar questions?
Why would anyone need five shots of .500 Mag? How is an 8-shot N-frame .357 necessary? What makes six shots of .44 necessary to begin with? Is there anything that anyone would ever shoot a .44 Magnum at that couldn't be figured out in 2 or 4 rounds?
Not trying to be obnoxious, I just can't understand how "six rounds of .44 Mag" is some kind of baseline for what is sensical or needed or proper... but a 7th is preposterous...?
The difference is that the 5-shot S&W Model 500 came with 5 shots by default, and was likely made that way because it's a revolver which tend to come with 5-6 shots on average.
As for 8-shot N-Frames, I believe that the first 8-shot N-Frame .357 revolver was requested by Law Enforcement, which makes sense given that Law Enforcement would want a high capacity revolver in a potent round such as .357 Magnum.
Same as the Model 500, the Model 29 was designed with a 6-shot cylinder because that's the capacity folks expect to see in a revolver and that's the configuration Smith & Wesson has long since mastered.
That being said, is the 5-shot S&W Model 500 practical? Honestly no, and I doubt that anybody would claim that the Model 500 is by any means practical. In reality, the Model 500 only exists because S&W wanted the marketability and bragging rights associated with the title of "The most powerful handgun in the world!" Same goes with the S&W Model 460 being dubbed "The highest velocity handgun in the world!" and is offered in barrel lengths which exceed that which one generally considers to be synonymous with handguns.
Both firearms were/are marketed based on hype alone, not because they're practical. They're extremely large, extremely heavy, extremely powerful, produce extreme muzzle flash, recoil, and report, thus making them extremely impractical in just about every role because they're excessive for just about everything.
Don't get me wrong, I have nothing against firearms which major selling point is their cool factor. (Heck, I carry a PPK/S and my Home Defense firearm is a Taurus Judge Magnum.) I'm just telling it like it is, an 8-shot X-Frame .44 Magnum is less practical and more hype.