This process is not a lot of extra work if you want your block EXACT. Consider all of the steps to making a block: Cutting, sewing 1/4 inch (precisely), pressing the block (not ironing, for God's sake!) After all of this, not matter how meticulous you are in your process, sometimes the block is just not spot-on. Maybe the fabric is is not of excellent quality, and it stretches a bit. Maybe your 1/4 inch is off a bit. Maybe your cutting was not exact. By making the pieces larger, when you're done, you can cut the block exactly to size.
Remember: Quilting is a wonderful journey, and each step is as important as the next. When everything comes together at the end, and you have a wonderfully executed finished product, all the painstaking care, (and sometimes seam-ripping anguish) is worth it! Enjoy the journey!