What is salience?  Fundamentally it is a value judgment that determines where your brain will place its limited resources.  There are a lot of things that could draw our attention.  The world is full of sights, sounds, smells, pressures, temperatures, stretches.  Our mind is full of thoughts.  Without a salience network, we wouldn’t know what matters and what doesn’t.  We would just randomly scan our thoughts and the environment and hope what we are noticing at any point in time is what will help keep us alive.  That is a losing evolutionary strategy.