Quilters typically use rotary cutters because they can make straight lines easily, enabling you to make lots of squares of the same size. They are used on top of a self-healing mat, which has lines in inches and at angles, thus helping you cut. If you lay a metal ruler on top, you can use it to keep the edge of the cutter stable and make a straight line. Another key to using a rotary cutting tool is to make sure the blade is sharp. If it isn't, then you end up scoring the fabric and need to go back over it with scissors.
This is a great tool when making small items (anything that will fit on the mat). The tool easily does angles and curves, and is an asset to any seamstress.