Reference has been made to articles on iguru.gr that the Windows Defender ranks as one of the top antiviruses worldwide https://iguru.gr/2019/07/30/av-test-windows-defender/ and now continues to prove it in every test https://www.av-test.org/en/antivirus/home-windows/windows-10/june-2020/microsoft-defender-4.18-202416/ with excellent in all areas: protection, performance, ease of use.
In the "performance" sector, it participates as a measure of comparison since the competition is now required to prove that it offers effective protection without negatively affecting the performance of the operating system, compared to the actual Windows 10 operating system with Windows activated Defender...
Also Windows Defender is part of an integrated protection platform that includes the Firewall (Firewall) of Windows (Security Suite).
The question "why install a third party AV” has no answer (so far) to home users and rather the answer is based on a status quo mentality of a large portion of Windows users who are impressed problems older versions and tend to adopt leveling opinions, "urban myths", the just the habit of so many years (install antivirus).
In this case:
The protection offered by an antivirus does not only lie in identifying all known threats, but also in the speed of detection and response to threats before the user associates any action with them.
The following test free / trial versions of the most popular antivirus of 2020 will help you understand how they work and the difference in reaction speed and protection of their Real Time Protection machines.
Zone Alarm Free
Eset antivirus Premium Edition
Free Panda Antivirus
Kaspersky Free Antivirus
Avira Free Antivirus
AVG Free Antivirus
And finally Windows Defender