Why do I still need the dmg?
For most people, in most situations, the answer is as you've assumed--no reason at all. If you do need to reinstall for some reason, you'll just download it again, probably a newer version at that.
As for those of us who do keep a copy around, it's a combination of paranoia and convenience. The two times I use them are if I need to reinstall for whatever reason (this would generally be a full-OS reinstall, not just the app, as there's no reason that would normally have issues), in which case it saves time downloading everything again (though I might anyway to make sure I have the latest version), and because I have three computers in the house, so I can always grab whatever the latest version I've downloaded on one system to install on others (I use a file server, where all the .dmgs end up).
The one other reason is that if you have a lot of apps, and you do a fresh system install for some reason, the folder of .dmgs is a handy reminder of what all you had bothered to install before--I've a few times forgotten to reinstall something until later when I went to use it.
There are also those increasingly rare times when the developer kills the product, and you actually can't find a download when you want it. Have a couple of apps that were EOL'd that I was glad I kept copies of, just in case.