To make a sewn project look perfect - the obligatory use of an iron is even more important than having a good sewing machine or the ability to make a straight stitch. Sewing creates a three-dimensional structure, whether it is a straight stitching, zigzag or serge/overlock. To get the best look, it is important to smooth EVERY seam ironing both woven fabric and knit/jersey, as you have to flatten it.
Also, iron can:
correct the unevenness of the seam, smoothing slight gathering;
reduce the seam-allowance volume of sewn materials;