Breadth is just a fancy name for electives, just as Ferrari is a fancy name for a car. It's essentially a marketing creation.
Most generalist degrees "force" you to diversify a bit. In first year science for example, you can't just take chemistry, that would only be 2 subjects, you need 8. So, you might choose some physics or biology or something like that.
Breadth at UoM takes it one step further i guess. They believe they're training you for more than a job like a tafe would, they're training you to think, to be an intellectual and broaden you mind. That's probably some of the idea behind it. Granted, its not a choice but they recently changed it to allow some room (i just totally made these numbers up but say you used to have to take 6 breadths, you now have a choice between taking 3-6, the numbers aren't real but they put a bit more flexibility into it in the past few years, rather than a set number everyone has to take).