When you change the configuration of a network router, there's a good chance that the network will become so messed up that the router is no longer reachable over the network. This serial port gives you that peace of mind - you can always connect to your router through it. I use it with my Ubiquity EdgeRouter ER4 and it just works. Note that you need to set the options correctly in putty (or a similar terminal). But if you're dealing with professional gear, it shouldn't matter much :).