Understanding how Git works and what exactly you are doing

GUIs are helpful but they add an extra layer of abstraction

Getting answers online

the official documentation and most of StackOverflow answers explain things in terms of the command line

Learning advanced techniques and concepts

valid especially for branching and merging

Automation

writing and using scripts so that the team can actually focus on solving problems

Unified interface in all environments

tools come and go; the CLI will be there, on every machine that can run Git