+1The point about singing when soloing on bass is that it can be hard to translate what's in your head to the instrument. Singing helps this process by guiding you to play what comes a lot more naturally with your voice - melody. I used to solo with little phrasing; then I started focussing on phrasing and sometimes singing a line in practice then duplicating it as closely as possible on the bass immediately afterwards. Once you get the concept of phrasing and melody, your solos will take so much more musicality.
Hope that helps.
Well said.
Bassists (& guitarists) tend to play a lot of licks or fretboard patterns. Solos played that way will work - they'll be in the right key and may look & sound flashy, etc - but they won't be very original or authentic.
By singing, you can free yourself from the limits of what you know about your instrument and find your own musical ideas. Then you can learn to express those ideas through the bass. That's the journey we're all on ...