From what I can see, it just appears to be a standard email client. So I don't know why you would use this over something that's open source. And with something like Proton Mail, this would not work if you are a user of that service.
I am using eMclient for several years now. I would have loved to use open source, but experience of using Thunderbird was unbearably sluggish and glitchy. Other options for windows were simply lacking. EM is far from perfect too, but still better. I hope to retry Thunderbird in upcoming years to check if things under the hood changed much.
Just took the opportunity to try Thunderbird again and had to downvote myself :)
I guess my previous frustrations were related to initial sluggishness while downloading and non-compacting loads tens of thousands of mails from gmail. It still takes some time, but after downloading and compacting my data, search seems to be very responsive.