One reason for hygiene

You need hygiene because:

Only superstars on their highest alert level [while writing macros] get things right all the time.

–Matthias
That said, hygiene alone isn’t a cure-all for writing predictable macros. If you don’t understand what you are doing, thoroughly; you will screw up eventually. “Thar be dragons here” as someone once warned on the PLT list.
(via PLT)