Because there's no vacuum assisted brakes if you need to brake and you had just been accelerating and making boost.

 

So this has more to do with turbocharged engines?
GM Yukon for example is NA engine, but still has vacuum pump driven via pulley and belt (stupid design I think):

These things don't appear to be 'plenty reliable', even our Volvo one typically starts leaking oil at around ~40-60kmi maybe?

 

Volvo (and other manufacturers) have used vacuum pumps for longer than just the I6 motors. They are plenty reliable.

 

Volvo has used them on naturally aspirated or turbocharged engines for a while.

Yeah, they leak but I was referring to them producing vacuum when I mentioned reliability. The electric ones would sometimes seize and blow the fuse or the vacuum switch would come apart. Resealing an I6 pump is cheap compared to replacing the others.

 

Agree. Our XC70 has only 30kmi, vacuum pump is a little damp - enough for dirt to collect on it at this point. I'm keeping an eye on it.

 

If you look in to performance car options for adding a vacuum pump to cars with poor engine vacuum,( big cams), several pics of the vacuum motor used in these conversions look very much like one from a Volvo.
Not sure they are related, but automakers seem to be removing as much as possible from the engine to limit parasitic drag and increase mileage. An electric vacuum pump probably creates more stable vacuum and allows brake systems and other vacuum reliant systems to operate without being influenced by engine performance