Filling the song is actually quite a good way to think about it. It’s not about loudness, it’s more like the foundation of a building. You need to be able to build *on* something.
For example, imagine [this song](https://www.youtube.com/watch?v=IuezNswtRfo) without the bass. Without the bass there is no song.
In club music you get 16 beats to a bar, for most of the track you get 12 beats of bass then the last four are a recognisable buildup then a bang as the first of the next 16 beats. This is a consistent pattern that is used to know when to cue up another to mix in or out to. You can use head phones listen to the alternate track whilst counting the beats of the track thats playing and as not all tracks are recorded at the same speed (or played at the same RPM) the bass then is a distinct marker that helps to know if one track is going faster or slower.
Depends on the music you are listening to. Most pop songs will have the bass ride the root notes. Bass players job is to keep time and out line the chord. So that in genres like rock, jazz, blues, etc the other instruments like piano, guitar, horns can solo and improvise. Jazz music is a great example. Bass “walks” the chord progression while keep time with the drummer, keeping the overall feel and rhythm while the improvisation is done.
Check out Miles Davis “So What”