Why use 2 eggs in noodles instead of one

It changes the texture of the pasta. Depending on what you are making, you may want the extra fat from the yolk. There are versions of pasta with one egg, two eggs, an uneven mix of egg yolks and egg whites, just egg white, just water, etc.

Ultimately it's the pasta cook's decision.

For a more detailed and learned explanation, please read the pasta chapter (about 35 pages) in Paul Bertolli's "Cooking By Hand" (2003). He has about eight base pasta recipes, each one with different ratios based on what he intends to do with the dough (wider noodles, raviolis, etc.).

Remember that not all flour is identical. Depending on what your final vision is you might vary the water/egg based on the type of flour you are making. While a recipe is a good starting place, it's not gospel. I always judge my pasta doughs by feel.

Best of luck.