Was this done to keep close to their Quattro history? To provide better ‘AWD’ in a sporting manner, while taming torque steering in the front end?

Is this setup designed for boosted engine applications, where a source of good vacuum is not available?

If your engine won’t deliver sufficient vacuum for a brake booster to work reliably then yeah, go this route. But ‘power brakes’ were developed many decades ago as a safety thing, and still provide that extra measure of safety today.