Here's why:

* Structural Components: Proteins are the building blocks of cells, tissues, and organs. They provide structure and support, forming things like muscles, bones, skin, and hair.

* Enzymes: Many proteins act as enzymes, which are catalysts that speed up chemical reactions in the body. These reactions are essential for everything from digestion to energy production.

* Hormones: Some proteins act as hormones, which are chemical messengers that regulate various bodily functions like growth, metabolism, and mood.

* Antibodies: Proteins play a crucial role in the immune system by forming antibodies that fight off infections.

* Transport: Proteins can transport substances throughout the body, such as oxygen, nutrients, and waste products.

While other functions are important, building and repairing tissues is the most widespread and fundamental role of proteins in living organisms.