You've probably heard the stories of Beethoven composing symphonies on walks that he took. While most people seem to think that means that he just made stuff up out of whole cloth when he was out in the Viennese woods, I think it's possible that some of it comes down to him taking basic ideas and mulling them over in his head - - working out themes and forms, then returning to his piano or music paper and playing or notating those ideas.
I think he was doing what I call auralizing.
I'm sure you're familiar with visualization, it's the same basic idea, but auralization has to do with hearing musical idea in your head. People often cal this playing or writing what you hear.
That's all fine and good, but what does it have to do with US?
Well, I think that it's really important for all sorts of stuff in music. From tone to phrasing to soloing to writing - - in my mind, it's all about using one's mind - - using one's imagination.
For instance if you can hear the tone you want in your mind, or can describe it in words, I think it's more likely that you can achieve that sound.
Likewise, if you're away from your instrument and try to imagine (hear) what you want a solo to sound like, my feeling is that you will be that much closer to really being able to achieve it.
I think that same goes for phrasing or any other part of playing an instrument, if you can auralize it, you can learn to play it.
It's a great way to come up with bass lines (or any other line) that do not rely on your hands as the main creative element. Everyone's fingers fall into patterns, or habits, why let your ideas be driven by that instead of by your imagination (which probably has better ideas than your fingers)?
Further, I think auralization is crucial to composing music. Hearing the basic melodic shapes, rhythms, form, durations, etc., BEFORE you start putting pencil to paper or fingers to your writing tool (bass, piano, synth, etc.) is something that I think is of the utmost importance. Again, we have our digital habits and I think that our imaginations are more important to creativity than our physical limitations.
Of course, auralization is just another "muscle" that needs to be exercised and grown. After all, you probably didn't just start playing the way you do now when you first picked up your instrument, you really had to work at certain skills like rhythm, intonation, etc. I think it's basically the same thing, except that you have to train yourself to really listen to your imagination. It takes a certain amount of discipline (which I don't always have!), but I think it's well worth the effort.