There's a huge difference when a driver first "signals" before they venture into your lane. You are aware of the intention in advance and can adjust accordingly. You are able to speed up, slow down, or "block" all while both drivers continue moving forward. Linking to your career, if you are one to offer solutions, opinions or examples outside of your area of expertise, it's nice to "signal first". This common courtesy allows for forward movement in a more collegial and professional manner.