Peter Cohen, iMore:
So Apple has blessed the use of the Mac Pro on its side, but the question is why? In short, Apple is guiding for the use of the Mac Pro in server settings.
Another really great use for a side-mounted Mac Pro: the biggest live concerts are at least partially run off a Mac Pro. The computer is usually mounted on its side in a flight case, and the old Mac Pro was perfect for that. This new one will require a much more comprehensive custom solution for securing one in a flight case.