The goal is something that fits your hand in a design you like. Everything else said on here can be true or BS depending on each maker.
Factory knives are mostly heavily made with a lot hands doing the work.
Custom knives are generally one offs.
You can get a custom knife to fit your hand. If you have a good smith.
Maybe the metal or hardness will be better.
Buyer beware. There are a lot of production $300 knives that are better than custom $300 knives.
I don't like attitude with makers, they are not rocket scientist saving the world. They get really busy and people think they will solve all lives problems. They just make a tool. Some of them are easy to deal with and others are just dreadful. Some guys get kind of an attitude about their products, or they don't answer request. Or they don't make good on an order. Or they make a crap product.
I am not going to trash a bunch of guys. I mostly quit buying American custom knives. I am enjoying buying hand made Puukkos in Finland, Norway and Sweden when I travel. 99% of them have only been out of the sheath a couple of times. I bought some neat ones in Finland last time, and I'll get myself a couple of new ones this year (probably next year too). Love Arctic Finland.
A knive is a tool. We have used knives in some form of fashion for our entire existence. I have some of my father's knives, and they mean something to me. I gave my 85 year old dad a top end $400 custom knife about 3 years ago. He will probably never use it. But he has it on his counter and messed with it. He says it reminds me of him.
They are a cheaper totally custom thing you can get for $200-20,000 that is hand made. You can't say that about most things.
