曲阴影中的主阴影为什么要相对定位？是为了z-index吗？

.effect{

position: relative;

box-shadow: 0px 1px 4px rgba(0,0,0,0.3),0px 0px 40px rgba(0,0,0,0.1) inset;

-webkit-box-shadow: 0px 1px 4px rgba(0,0,0,0.3),0px 0px 40px rgba(0,0,0,0.1) inset;

-moz-box-shadow: 0px 1px 4px rgba(0,0,0,0.3),0px 0px 40px rgba(0,0,0,0.1) inset;

-o-box-shadow: 0px 1px 4px rgba(0,0,0,0.3),0px 0px 40px rgba(0,0,0,0.1) inset;

}