It is the difference between buying a shirt or suit (for men) that comes in Small/Medium/Large and buying one that is designed for a certain body type (broad shoulders, slim waist or short but fit) initial design. You can alter both, but a design that is more specific than S/M/L will usually look alot better after altering than a generic one.
I have both range of suits, and I can say there is a world of difference, even after altering between high end suits in fit than generic ones.
A very specific example a couple years ago when I got married, I ended up buying a really nice suit. That year trend by brand: Zegna suits were usually designed for the average man (height/weight), Ralph Lauren for more short guys and Armani for tall slimmer folk. As far as I've remember buying a suit, RL suits were always the better fit before and after alterations since I'm short, specially their more italian line that goes for short/broader shoulder/slim men.
