If you knit one round and purl the next you would get a garter stitch. The same as if you are knitting flat and knit every row. The back and front would look the same. If you are doing a stockinette stitch and you were knitting flat you would have to alternate each row-one row of knits, one of purls. One side of the fabric will have neat little knitted Vs all lined up, and the other side will have have purly bumps. If you were working a stockinette in the round you would knit every round, your Vs would be on the outside of your tube, and your bumps would be on the inside.
I definitely prefer to knit in the round, right now I am making my first scarf and this back and forth is killing me. I debated frogging the whole thing and knitting it in the round then seaming the two short ends together to make a double sided scarf, but I am already forty rows in and I don't even know if I have enough yarn for that.