Reference has been made to articles in iguru.gr how 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 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 a complete platformof protection which also includes the Protection Wall (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 lies not only in the recognition of all known threats, but also in speed detectionand dealing with threats before the user associates any action with them.
The following tests of free/trial versions of the most popular antiviruses of 2020 will help you understand how operation them and the difference in the speed of reaction and protection of Real Time Protection their machines.
Zone Alarm Free
Eset Antivirus Premium Edition
Kaspersky Free Antivirus
Avira Free Antivirus
AVG Free Antivirus
And finally Windows Defender