You would only need a height if the parent had a height defined
At present it is the content creating the height so it will stretch the parent automatically so parent and child are always the same height.
If you set the parent to height 300px for example then the content would only be 100px high and the background would only change on the content and not the whole parent. In that case you'd need a height on the child also to match.
Most times you would want to avoid setting heights unless you are trying to make a number of elements the same size. the problem with setting a height is that it can never grow and if text is resized or content increased the content will overflow.