个人理解,仅供参考:
1. The condition of floor and sub-floor can change over time. After a few years, sub-floor can warp along joist. If a floor is not installed perpendicularly to joist, it may show curve/crack, and cause squeaking. It will be hard to fix during that time.
2. Some floor manufacturer may require you to install this way to satisfy warranty.
Some people try to avoid this kind of installation by using a thicker sub-floor. However this approach may cause other problems, and is subject to building code.