PJ
You use the circular needle because it holds more stitches. It would be really hard to knit with 18" needles. You can cram more stitches onto a circular needle. Just knit it like you would a scarf, do a certain number of stitches, and then turn the work around and work back the other way. If you were knitting in the round you would not turn the work but increase to the next row and keep working in the same direction. Some people use stitch or row counters. I never found those to be worth the trouble.

vnelson85
When making a blanket, its more comfortable to use a circular needle. this way you can still work back and forth like you would on straight needles, but this way you can hold more stitches comfortably and still have room to work.