There really is only one reason to buy a custom knife: because you can't find what you want in a production knife.
Custom knives are just that: custom. They are made to your specification. If you choose the right maker and communicate your desires well, you can have your knife just about any way you want.
Some custom knives are deliberately made more as works of art than as practical knives. You'd be a fool to carry one of these knives around and beat it up in daily use.
Some custom knives are made to be extremely strong and durable. They are made for brutal use.
Consider, for example, a kris blade. It's not the best blade profile for the tasks that most of us use our knives for day-to-day. And it's a nightmare to sharpen. But, they are an excellent fighting knife and they're also wonderfully attractive and artistic. So, if you're looking for a great fighting knife or if you're looking for a great piece of collectible art, then maybe that's something you might like to think about. Unfortunately, there aren't any good ones in mass production. So, you'll probably have to have it custom made.
Somewhere on my site there's a picture of a Pacific Cutlery custom tanto. It's been its owner's daily carry for utility purposes for getting on twenty years now. And he tells me it's in excellent shape even after all of that use and abuse. Of course, it has classic skeleton handles in Titanium. He ordered it specifically with the intent of using it as a daily-carry utility knife.
On the other hand, only a fool would plop down $800 for a BM49SPL and then use it as a daily-carry utility knife. Aside from the issues with the kris blade which are amplified by the zero-bevel grind, the highly polished surfaces would be quickly scratched up and the wood inserts would also suffer nicks and dings and scratches.
So, the point is that you can go either way on custom knives. If you don't see what you want in an off-the-shelf knife, whatever that might be, then you should consider a custom.
If what you want is an exceptionally rare or exceptionally beautiful knife to add to your collection, then go for a custom.
If what you want is an exceptionally durable knife with the exact blade profile and exact size, weight, and latch configuration that you prefer as the perfect daily-carry utility knife, then go for a custom.