During primary the yeast are active using O2 if available and out gassing CO2. This prevents and oxidation problem.
During long term aging the beer will oxidize if exposed to much O2, which will lead to off flavors.
PET is nearly impermeable to O2 while food grade buckets are quite permeable.
So PET is good for long term storage of a stable beer or wine. It can be used for primary if you prefer to visually see the beer, and have a material less scratch prone but it is not necessary during active fermentation.
So O2 permeability is the main reason for using PET or glass over food grade buckets. For comparisons of PET and glass see the many threads on that subject