I’ve heard it said before that every time your hands leave the keyboard to use the mouse, somewhere a baby kitten dies. Okay, that’s a silly joke and an exaggeration. But spend any time with an expert in various fields in which a computer is a major part of the day and you’ll see the element of truth here.
Whether a designer or developer, someone working in the movie or music industry, or an intern for a larger corporation, experts learn to do their jobs more efficiently. You’ll find that he/she knows the shortcuts of their preferred application like the back of their hand.
Gmail is no different in this respect.