As a developer, we spend a lot of time working on the terminal and having personalized shell makes the working environment perfect, decreases frustration and also, increases productivity.
Figuring out how to fix the problems in Mercurial is near to impossible. Yes, you heard me right, and it's very easy to screw up. Mercurial documentation has this chicken and egg problem where you can't search for how to get yourself out of a mess unless you already know the name of the thing you need to know about to fix your problem.
I am fortunate enough to have them as mentors as they not only guided me technically but also showed how to become a good software engineer and human being.