I can't say I really understood the logic of making the default port 6346.

At the time it came out, one of the advantages Gnutella boasted was that you could change the port to whatever you wanted so that it couldn't be blocked simply by the network admin blocking the application port. This was a good idea.

Unfortunately, almost everyone kept the default of 6346. There is no real reason for this, since it works quite well on whatever port you use. Personally, I normally use port 80 whenever I run it, since I WANT my ISP to think I'm only running a web server.