I've thought a lot about this, too, and so I could be very wrong here.
Fit is part of it. For instance, I went from a JC Penney suit to a Hickey Freeman Madison & one thing right away I had to get used to was the high armholes (which meant I had to put the coat on a little differently) and the trimmer sleeves; the improvement in appearance was amazing even before taking it to a tailor. But this isn't universal. In some cases, one can have a suit made cheaply based on them receiving your measurements. So you can have a cheap suit with a great fit.
You might see other styling details on a good suit that you won't often see on cheaper suits, like double rear vents or functioning buttonholes--although some less expensive suits may have one or more of those. Some folks put a premium on hand-make suits, with hand-stiched buttonholes, primarily for reasons of exclusivity (there's nothing illegitimate about that, but the tangible improvements probably don't warrant the markup--see below).
But I think the real benefits you can't really see, which would be the improvements in construction. Full canvas, if I understand it right, means that the jacket will keep its shape for a long time. I think this means that over the years a cheaper jacket's fabric will hang rather badly. With a good suit, you won't have to worry about rain dissolving any adhesive holding the jacket together, either. So long as your own shape doesn't change, the suit will look grand on you a long time. Perhaps at cheap MTM or "bespoke" establishments, the construction can be inferior to a OTR Brioni, HF, or Isaia, and thus look like a cheap suit over time. So with a good suit, you will look good (after tailoring) for a very long time--the suit really does become an investment.