The walls of blood vessels have two layers of smooth muscle. one layer is lengthwise and the other layer is around the circumference of the vessel. why is this arrangement important?

Imaging the blood vessel as a simple thinned walled tube.If you put reinforcing rings around it it would retain its shape where the rings were, but in between the rings the blood pressure would cause it to bulge- it could also stretch lengthwise like a slinky.