It's always seemed weird to me as well. Let's say you go to the doctor for something like strep throat. You get a prescription for an antibiotic. Doctors offices don't carry drugs, so you take this prescription to a pharmacy to fill it. The pharmacy is a third party that isn't affiliated with the doctor's office. Only doctors can write prescriptions, and as far as I'm aware only pharmacists can fill them. Pharmacists aren't allowed to write prescriptions, even if it's something trivial.

On the insurance side, it's a nightmare. The doctor probably charges your insurance $200 for the office visit. Your insurance has a negotiated rate with the doctor's office for $150. You have a coinsurance of 50% if you've met your deductible, so assuming you haven't hit your max out of pocket for the year you pay $75. If you haven't met your deductible (some arbitrary amount you have to pay on top of paying for insurance) you pay $150. If you have hit this max out of pocket number, you'd pay 0. The $50 goes into the void. My employers insurance is something like a $1000 deductible / $5000 max out of pocket / 75% coinsurance.

When you get the prescription filled, well, I have no idea how this insane process works. Last time I needed antibiotics the receipt had a "before insurance" amount of $50 with an actual cost to me of $4. I have no idea where the $46 went or what my insurance actually paid.