I thought the included (pre-installed) Microsoft Windows Defender (or Windows Security Essentials) was already good enough.
Yeah, it pretty much is. The reason to go with something else, or in addition, is largely if you're in a business setting and you want to be able to push updates and monitor results. Also, I'm not sure about the current situation, but last I checked, MSE was free for personal use but not licensed for business use...?
But for home use, MSE is probably good enough. It also doesn't have popups, it doesn't break any apps or anything in the OS, and it doesn't take up tons of system resources. Ultimately, with AV software, you're choosing the lesser evil: having the AV take up system resources and break things is less evil than the viruses themselves, but if you can find an AV that doesn't do those things, go with that.