`lh` and `rlh` units

There’s some new units I was totally unaware of from the Level 4 spec for CSS values! The lh unit is “equal to the computed value of line-height” and rlh is the same only of the root element (probably the element) rather than the current element.
Why would that be useful? Šime Vidas’ has a strong point:
“Vertical Inline Centering” of an icon
.inline-icon {
display: inline-block;
width: 1lh;
height: 1lh;
}