Is Port Forwarding Necessary?

I recently switched to a cable modem with a built-in router. The router doesn't allow port forwarding -- the box came from Time Warner and evidently they don't want me doing this. But it doesn't seem to make any difference. I can search and download, and others can upload from me. So should I do anything? I could buy an external router, then disable the routing functionality altogether in the cable modem (they do allow that) -- but is there any point? Thanks in advance.

Your router maybe doesn't allow manually configuring the port forwarding, but since things are working, apparently it supports UPnP or NAT-PMP, and you don't need to do anything special. The client uses one of those protocols to ask the router to automatically set up port forwarding as needed. If you were to disable both in Options > Login, probably things would no longer work.

The only advantage to doing it manually is that sometimes the UPnP or NAT-PMP request can fail to open a port for some reason, or the forwarding can stop working after a while (e.g. if your router resets unexpectedly, as mine does when my household power lines have a mini-brownout). If you have manually configured the router to always forward that port, it stays open.

Most routers allow you to disable support for those protocols if you want, and some come with it disabled by default. The advantage to disabling it is that malicious software on your LAN can't set up port forwarding automatically. The disadvantage is that neither can anything else, so you'd have to manually set up whatever port forwarding each server needs.