Try to do a Mariner's Compass without paper piecing by machine! You'll soon find out why to learn paper piecing. Some people paper piece because they want perfect pieces, every time. Sewing on a line gives new quilters the accuracy of seasoned professionals.
As a test you could try cutting half square triangles and sewing them together to make squares. Now try the same thing using a paper foundation. Perfect squares, the same size, every time. Imagine working with diamonds! Bias on 2 sides to get all wonky on you when you least expect it.
Once you conquer the quarter inch seam and proper pressing, paper piecing is usually reserved for blocks that require pointy points--stars especially.
Another reason for paper-piecing is to hold wonky fabric and bias edges steady. It's great for stabilizing scraps to make string quilts or crazy quilts.