Do I need to buy antivirus software if I have Windows defender?

The built in windows defender and good habits are fine. Third party antivirus programs have to do a man in the middle attack on the system to even work. This has led to them being virus vectors. Avast for example, once lost the secret part of their certificate and the hackers used that to infect avast protected computers.

You need something more aggressive than Windows Defender Windows Defender often lets things get by

In tests, Windows Defender has been demonstrated to miss some of the viruses and malware for which more robust packages provide better protection. Of the free packages, tests have shown Avast catches problems that Windows Defender misses. Some of the paid packages provide better protection than the free ones. You should read as many reviews as you can find and then make your own decision.

Webroot is garbage, uninstall it.

If you're not clueless on how to use the internet, Defender is more then enough.

If you have kids on your PC or someone who likes to click every link they come across, you might want to get AVG or something else.

Many good av softwares are free. Avast, avg