Free antivirus software from Microsoft

I never understood why Microsoft would leave the job of securing their OS to 3rd parties.  They recently made available a free antivirus program called Security Essentials. Not to be confused with their Defender program that has been out a while, and that only dealt with malware.   I had been using and recommending the free and capable Avast program but I am now migrating to Microsoft’s offering.  Not that I know it is better but I am hoping it is good and it is more convenient and integrates better than Avast.