It has been mentioned in articles on iguru.gr that Windows Defender is ranked as one of the top antivirus 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 field of "performance", it participates as a measure of comparison since the competition is now called to prove that it offers effective protection without adversely affecting the performance of the operating system, compared to the operating system Windows 10 with Windows Defender enabled…
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 domestics users and rather the counterargument is based on a status quo mentality of a large portion of Windows users who have been impressed with problems of 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 recognition of all known threats, as well as the speed of detecting and dealing with 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