I've had several computers crapped by malware and hacking. Something is always better than nothing.

and don't see why anyone would need it.

without having to install anything else. If kept up to date, both
windows update and Defender, you should be protected from most
things if you don't somehow allow access. If you have a router,
it should have a firewall in it as well.

... exactly right. Back in the day you needed third party firewalls but believe it or not, Microsoft really got their shit together with Win10 as far as firewalls go. The built-in firewall should do just fine. As 5X stated your router is highly likely, almost guaranteed, to have a built in router too. Lol, I'm still not completely sold on Windows Defender though and do use a third party A/V. By all accounts though, its much improved and more than adequate for the typical user.

Zone Alarm is crap. I use Windows firewall, the firewall in my router, and Avast. Windows anti-virus uses too much memory and can slow the computer if I have more than a couple of programs running, so I leave it turned off. Avast has a fine reputation and a lighter footprint. I run a Malwarebytes scan from time to time to be sure I'm clean.

My computer was rendered useless with Trojans/worms/viruses. You name it.

The ZA people told me what to do. I needed to make my own router a trusted site. We'll see if it works.

.... this the wrong way please, its not meant as a slam. But if your machine was "rendered useless" then you really need to practice safer surfing and computer habits. I go to some mighty iffy sites on a semi-regular basis and haven't have a Trojan/Virus in years. Don't open things until you scan them, look at files etc. before using...... if you think its a .jpg it should NOT have a .exe extension for example.

And only Windows Firewall, and used to visit a lot of sketchy sites, d/l from torrent and usenet ... simply paying close attention to extensions, and checking that file sizes match what is appropriate for what I *think* I'm getting (movies are not 128KB), and not clicking on links w/o knowing where I'm going (hover over them and the browser will show you the destination URL) and I *RARELY* picked up anything on my PC. Maybe twice in like 5 years of PC use every day w/no AV.

And the past like 8 years, since Win7 with Windows Security Essentials/Defender came out, wherein I manually scan anything I d/l (that I'm unsure of) before running, and use the weekly scan, I've had no issues.

Granted I'm a very experienced user, but the built in windows stuff nowadays + just a modicum of 'attention' to safe-surfing basics (and not letting 'kids' on your PC lol) should be enough for all but true novices, IMHO.