Cooking means there is heat outside the meat making the meat hotter (and breaking down proteins, of course, that make cooked meat different from raw meat even after it has cooled again). When a cut of meat is thick, cooking becomes more of a challenge: high heat will cook the outside of the meat correctly but the inside not so well. And if you continue to cook until the inside is cooked correctly, you can end up with a section nearer the surface that isn’t as palatable.
Barbecue smoking and sous vide cooking both seek to minimize that issue by cooking at lower heat for longer periods of time, so that the outer and inner portions don’t differ dramatically.
But any cook is made easier when the cut of meat is (a) uniform in thickness, and (b) not very thick. So butterflying a cut, or spatchcocking a chicken or turkey (same thing as butterfly: you use shears to cut out the spine and then flatten the bird before cooking to create a more even and quicker cook) is just a technique to make getting an even cook easier.
(Off topic, but sous vide delivers an even cook beautifully, but because the sous vide never rises above the desire internal temperature, the meat’s exterior is never browned, which is not just a cosmetic flaw. The browning, or searing, creates caramelization of the exterior and a process called the Malliard reaction, which produces the desirable flavor. This is why sous vide meat needs to be seared after the cook is finished.)