For me, it was mainly these points that were important:
- manage multiple email accounts
- easy to use (I want to concentrate on my emails, not have to struggle with the program)
- HTML email signature for business emails
- free of charge or without subscription model
What always bothered me about Outlook was that it is incredibly complicated to integrate its Google contacts. It is possible, but you have to install third party add-ons (I had used gSyncIt). After I wanted to synchronize my calendars with the same tool and then copied and synchronized all my appointments, which I enter neatly into several calendars to separate private from business, my patience with Outlook was at an end - something new had to replace it.
I use Windows 10, so the idea of using the integrated "Mail" was obvious. This does what Outlook can't do - connect to Google. Just enter the data, everything is there. Multiple accounts are also no problem. Too bad that Mail does not support HTML signatures.