Let’s first talk about ironing vs pressing.
Ironing is the act of removing wrinkles. This can be on fabric you are yet to use, or a garment that has already been constructed.
Typically, when ironing, you continuously run your hot iron over the fabric surface. Sometimes you apply steam, sometimes you don’t. But your iron is in motion.
Pressing is different.
Pressing is placing your iron on something for some length of time. And then moving it to another place if needed. Generally, when pressing you are creating a crease in the fabric.
Sometimes that crease is at the seam (like when pressing seam allowances open). But it doesn’t have to be. You could be pressing pants with a crease down the front or back of the leg.
The quality of appearance of your final product will be directly affected by pressing.
When I first learned to sew, I balked at pressing.
It seemed so tedious!!! Gah!
But the reality is, if you want your schtuff to look professional, you need to press… things.. (Usually seams) while you sew.
Additionally, some things just go easier, if you pressed the correct thing the right way in the previous step. This won’t be obvious till you sew a bit and start recognizing where pressing has helped.
Seam allowances going in the “correct” direction, clear hem distances, smooth curves are all accomplished with good pressing.