The purpose of the purl stitch when knitting flat is to place all the bumps on one side of the knitting so that the fabric appears flat on the front. A purl stitch is just a knit stitch done backwards. You notice how you go into the purl stitch in the opposite way, and how the yarn is in the front instead of in the back? It's just knitting backwards. The same effect can be achieved if you knit right and left handed and never turn your work and just knit forwards and back across the front of your work.
SO! Why do you only knit to get stockinette in the round? Well, because you don't NEED to put all the purl bumps on one side, because you don't need to go back and forth, you're just going in a spiral and thus always in the same direction. However, to create garter stitch then you have to purposefully put the purl bumps on both sides by knitting one round and purling the next and alternating like so.
I hope that helps you. :D
I'm a boy, by the way.
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