What if you can't get to it? If a port has been captured while you're away, which often happens, you don't want to spend an hour or two sailing to the other side of the map to what is now enemy port to close an outpost you can no longer use anyway. Also, many of us are at our outpost limit, and we need to quickly shed old outposts to make new outposts in a pinch to participate in dynamic game play events. This became even more important with "patrol zones" where we now need to have a bunch of free port outposts, and then frequently delete them later on at times when we have other needs.
I always consider "sinking Tbrigs" to be wasting my time.