chemistry, alchemy, artistry and all points between...
I am not a perfumer. I am critically dependent on them. They take my formulation and do three things that correct and stabilise:
Chemical: there is skill, craft, knowledge, experience, science and mystery in the chemical component of fragrance creation - the chemical reactions and interactions of the constituent parts. I've witnessed alchemical magic where something that apparently cannot be apprehended by our sense of smell is used with dramatic effect in a fragrance. Perfumers are the alchemists, the gnostics, the witches and priests of scent.
Legal: many times, a perfumer will look away from their screen and tell me, "you can't put that much of that in there". Law and regulation, mandatory and voluntary, create boundaries in the fragrance industry. I've learned so much from Perfumers in terms of what you can and can't do from this perspective. I generally learn it and then forget it when I'm in a formulation phase, knowing that the best perfumers can help navigate the compliance using their art.
Aesthetic: many times, a perfumer will look away from their screen and tell me, "you can't have that much of that in there, it doesn't smell right". My initial reaction is usually less than humble as I'm very attached to the formulation at that point. Like a favourite film you've watched so much it can be viewed with the sound muted, after so long formulating and compounding, I "hear" things in the noise of the fragrance silent to others. A great perfumer renders it audible.