Pretty much what disneysteve just said.
-Two factor authentication for many essential websites (banking, investment, etc)
-Talking to my best friend and family on WhatsApp
-Checking FB, Twitter, and email
-Checking WSPR.net and the Reverse Beacon Network for my signal (ham radio)
-HamAlert app to tell me when my ham pals are on the air
-Paying with the Starbucks app
-Probably numerous other things that I can't recall right now
My two cellphones are an essential part of my life. One is used as a cheap mobile data hotspot. The other is my actual phone. However, anything that involves lengthy pieces of prose is done on the home desktop, unless it's not possible. I'm perfectly capable of writing quite lengthy posts and emails on my phone, but it's a more fluid and enjoyable experience on a full size keyboard and monitor.