What is the main purpose of proteins in healthy eating?

To get the small amount of each amino-acid needed with each meal for proteins to me made in your body (Note: Proteins from food CANNOT be used by your body directly they are always broken down into animo-acids first). The most important amino-acids are termed essential because you cannot synthesize them yourself if needed however it is best if you have a balance of every amino-acid with each meal as synthesizing amino-acids takes energy. Amino-acids are not stored (except as protein which isn't broken down except for recycling when not needed or as a last resort during starvation) which is why you must get sufficient amounts amino-acids with each meal in order to be healthy.

However please note that a normal adult (children need more as do adults putting on a lot of muscle which is protein) doesn't need much protein each day (a glass of milk will suffice by itself (but 1/3 at each meal as above)) in order to get the amino-acids he or she needs. Any excess amino-acids are used to generate energy and any excess energy is stored as fat. Protein contains nitrogen and this it is nitrogen from excess dietary protein that is excreted in urine (or nitrogen from body protein broken down for energy during starvation).