It's not particularly important.
My first serious build had one, subsequent builds have not. From time to time I think "Oh I could have used one of them" but not often.
Essentially it means you have access to the back of the motherboard - many CPU coolers, and most waterblocks have retention plates or nuts that have to go on the back. It also can make it easier to access stuff to give you a bit more room to work with. However if things get really tight then it doesn't help, since you need to allow space and cable length to slide it in and out.