For me what sets it apart is the ability for it to check 6 of my email accounts, collate all the mails in one place (all inboxes) or view them indivdually as I wish, along with managing imap folders well.
Whenever I want to send a mail, I do it all from the same place, in the same client. Just works as it should.
Trying to do this on Android was a non starter, the default email client only wanted to deal with 1 account at one time, plus run the gmail app to access my google accounts separately, and it wouldnt even collate those few accounts to one place, every inbox had to remain isolated. extra work every time I wanted to use a mailto link to decide on the sending account and program.
I could only realistically manage 3 of my accounts on my old android whereas my Iphone manages all I throw at it perfectly and in an user friendly manner.
My android was an older version, but when playing with current ones, my issues seem to still exist