The fabric frays much less, if at all for a cleaner look.
If you are making a flower, it will make it appear more full and drape better
HOW do you cut fabric on the bias?
Photo 1: Line up your quilting ruler at a 45 degree angle to your selvage edge (usually white with writing on it if you are using a cotton). If you are using a silk, you can just start at the bottom corner of your piece of silk.
Photo 2: Move your quilting ruler over to the left or right the number of inches you wish the width of your strip to be. For example, I moved the ruler over three inches so I could cut a three inch wide strip. Use your rotary cutter again along the side of the quilting ruler.
Voila! you have a strip with diagonal edges now!! The strip will be a bit stretchy which is a great benefit to cutting on the bias.