Your daily commute usually takes 15 minutes. But on certain unfortunate days, treacherous roads stretch it to 30 or even 45 minutes. Unless you want to be very late, you have to anticipate those days and budget extra time for yourself.
The same is true rehearsing music. You, the director, need to check the weather of the ensemble regularly, anticipate construction zones, and beware of icy patches. Do all that and you’ll almost always arrive to your concert on time, with the music prepared. Fail to check road conditions and you risk a catastrophe.