The chassis allows a whole lot more customizability to your body.
For me for an example my perfect length of pull is about 13.75".
Through a variety of different scopes, rings, if I want to run any sort of NV / up front optic, I'll have to raise my cheek piece to get a proper sight picture.
Depending on my shoulder/body I might want my buttpad a little canted. Depending if I'm shooting standing or in prone, I might want my buttpad either higher up or down.
For my hand to have a proper 90 degree trigger pull, I need to have my grip slightly closer to the trigger by about 1/4"
Additionally, most chassis are heavier and allow the addition of weights to reduce felt recoil. They also allow the weights to allow a better balance point.
I've run MDT's for a long time. Then I got a Manners but it just wouldn't fit me well enough because the grip was just too far from the trigger for me to feel perfectly comfortable. I had to add 1/4" of foam to the grip to move my gripping hand further forward.
I love the look of the Manners, but I'd need to pretty customized to get it to fit to me and they couldn't get it dialed in so I sold it and went back to my MDT. I just recently got a Xylo which I like.