Good fit - yes. Seat in the proper place relative to the bottom bracket. Handlebars to get the proper lean forward and comfortable arm bend, and for me, a stretched and elongated back. So I like my handlebars further forward and not especially low. I have long arms so my stems are very long.
I started the long stem journey when I was having real back pain and anaerobic pain in my torso on hard climbs on my commuter. It was a touring frame with a short top tube. I had a custom, very long horizontal stem made. One of my first rides was a very hilly 70 miler. No back pain at all! Hills were seriously hard but no pains that shouldn't be there.
I now compare my position on the bike as being like a cat. I'm at my best stretched like that cat when it stretches impossibly long. Yes, I'm exaggerating but you get the idea. I want my vertebrae stretched, not compressed, especially on long hills.