I have seen many threads where multiple people swear they don't use primer.
The reasoning is that aquarium use is such low pressure, there is no need for primer.
I tend to believe the argument as it makes sense.
Your correct but primer doesn’t just held weld the fittings together it also cleans the PVC from any oils and contaminants.
I always lightly sand, chamfer, prime and then glue. I have yet to have a leak and I have glued well over 1,000 fittings and pipe in my time.