Proteins are usually used for structural and enzymic purposes and only broken down when there is an excess or during starvation.
When there's an excess of proteins, proteins can be broken down and converted (depending on its structure) into acetyl-CoA, pyruvate or oxaloacetate. Acetyl-CoA is usually used for fat synthesis and pyruvate and oxaloacetate can undergo gluconeogenesis, making glucose in the process. However, fat is a much more efficient storage molecule.
During starvation, proteins are broken down as a last effort to provide nutrients, and the acetyl-CoA, pyruvate and oxaloacetate generated are usually used directly in the Krebs cycle. However, in the liver where more nutrients are stored, gluconeogenesis can still occur to provide glucose for the rest of the body.