So, what are some useful / important Android and IOS apps for developers which can help them on the go?

JuiceSSH for, well, ssh and other things
Termux and the like

For testing get Fabric, really easy setup (for iOS at least have yet to try it on Android) and a monkey could install it on their phone lol

Sololearn for great tutorials, funny and dumb questions and a coding playground.

Sketchware for easily making apps with a scratch like programming language. (Sometimes really fun to use!)

SixTeen for Binary and Hexadecimal Calculator (If you need it)

And of course there are a lot of tutorials about languages and frameworks to learn on the go.

Termux