Did someone actually say that no one knows why we need protein?
Proteins are the building blocks of human and animal structure. Proteins serve innumerable functions in the human body, including the following: formation of the brain, nervous system, blood, muscle, skin, and hair; the transport mechanism for iron, vitamins, minerals, fats, and oxygen; and is the key to acid-base and fluid balance. Antibodies that the body makes to fight infection are made from proteins.
You can certainly get your proteins from plant sources, but you have to properly combine them to get all of the amino acids necessary for synthesis.
All sources that are legit recommend between 10% and 35% of your total daily calories come from proteins, 45% to 65% from carbohydrates. and 20 to 35% from fats.
If you are athletic, you may need the upper end of protein for proper muscle repair, and the upper end of carbohydrates for energy requirements.
I've eaten in the range of 90-100 (give or take) grams of protein per day for decades. I work out hard, and am healthy and fit. I can give kids 1/3 my age a run for their money.
Yeah, I guess that's bragging.