For me two very important reasons.
First cutting out grains and startchy veggies allowed me for the first time in decades to get my appetite under control. It has been awesome beyond my imagination.
Second it finally allowed my body to start burning stored fat on a regular basis. I went from hating my metabolism to loving it.
Would I, me, myself have succeeded any other way? No I don't think so.
Note I did not give up veggies I eat more than before. But grains are gone, potatoes way down, and fruit very moderate.
It's what has worked for me. And many others. Found out about it from others on the Internet. But everyone needs to find what works for them.