Why do we eat protein when Rough Endoplasmic Reticulum makes protein itself?

Every day millions of cells are produced and old cells are destroyed in the body. All new cells are made of proteins and the body requires 20 different amino acids. Of these 11 amino acids are treated as non-essential as our body can produce them while 9 amino acids are called essential as our body cannot produce them and we must get these from the food what we eat. Our body goes through continuous wear and tear every day and the energy for this is obtained using nutrients like protein. Our body does not store excess protein, hence daily intake is necessary to ensure that our body gets all the proteins required for the proper functioning of the body.