Sewing machine needles dull after hours of use, and can also develop little burrs. Sewing with a damaged needle creates larger holes in your fabric, and if it is really damaged (burrs) it can bend as it goes through your fabric and possible break in your bobbin area. They can damage your machine or cause a problem with the timing.
When you hear a popping noise as you sew, you are overdue for a new needle. They should be replaced after each project or approx. 8 hours of sewing...sooner if you hear the popping.