I mean, sometimes cats need care and the vet doesn't really have time to gain their affection before poking them with a needle or prodding them all over. Not every cat is a fan of that, so sometimes you gotta deal with a cat who is trying very hard to claw your eyes out.
If you can't handle them without sedative then you can't administer the sedative in the first place. If you can handle them well enough to give them a sedative shot then you could just give them the vaccine shot you wanted to in the first place
I mean cats can be a lot harder to handle after you poke them. What if you need to give multiple vaccines? What if you need to draw blood? What if you need to touch other parts of their body to look for issues? What if you need to examine their mouth? Some cats will put up with it, some (a lot) will not. It's a lot safer and less stressful for everyone if you just use a sedative when needed.
Inhalation anesthetic. We had kitty sized masks for the cats that just wanted to hiss and spit, and we had a clear, plexiglass box for the cats that wanted to rip our faces off. Once the cat goes googly eyes, we could intubate, or just finish the procedure quickly, and get it back in a carrier to recover.
We also had a bag. It was basically a thick pillowcase with 5 zippers on it. We'd stuff the cat in, and only unzip one hole at a time.