When you started knitting, you probably learned on regular old straight knitting needles. You probably used them exclusively until you learned about knitting in the round on circular needles.
Then you might have learned some fun circular knitting tricks like knitting in the round with two circulars rather than using double-pointed knitting needles or even working two socks at a time on two needles.
Your use of circular knitting needles need not be limited to when you're working in the round. There are a lot of great reasons to knit flat on circular needles as well.
If your project is very large, whether in width or in length, you're probably going to want to work it on a circular needle. Once your project is wider than the length of your needle, it can get hard to work the stitches evenly or to keep them all on the needle.
Big projects, whether big in length or width, are also heavy, and knitting with straight needles actually puts more strain on your arms and wrists because you're supporting more of the weight of the project as you knit than you would be with a circular needle. The cable holds more of the weight of the project and allows you to keep it in your lap more easily than you can when working with straight needles.
Moving and Storage
Most of us don't spend a lot of time knitting in elevators, but if you're traveling with your knitting, no matter the size of the project, it's a great idea to work on circular knitting needles. A circular project is more compact; you can knit with smaller motions than you would on straight needles (which may also account for the increased speed of knitting), so you won't bother anyone who might be near you as you travel.
You also can't drop a needle and have it roll away from you because the needles are attached to each other. And when you decide to pack your knitting up, sliding all the stitches onto the cable instead of on the point parts is great insurance against stitches slipping off your needles in transit.
Working with a circular knitting needle while knitting flat is just the same as working with straight needles. Once you've cleared one side of the needle, you turn your work and start again with the empty needle in your right hand. Once you've used your circulars for a few flat projects, you'll probably find yourself reaching for them for most projects.