A bass guitar at the end of the day is a tool to create music. I justify my bass needs / wants by the fact that not all basses are created equal, so having a bunch is like a painters pallet of colours. But how many colours does one need? What if just one bass is the only pallet you really need?
Actually your assumption is somewhat flawed out of the gate. A bass guitar isn't just a "tool to create music" - it depends on the person. By the broadest definition, it is a physical object. It can be used to make music. It can be viewed as sculpture. It can serve as an investment. While the original design was to make music, like all objects, it can serve multiple uses.
That assumption aside, since you ask about needs, perhaps we should invoke Maslow's Hierarchy of Needs:
The bottom represents the critical human needs, and "Psychological" refers to survival needs - food, water, shelter, sleep, sex. Not clear a bass falls into that level of "need" unless you're *really* friendly with your bass.
The second level refers to security (personal, emotional, financial) and health. I could argue that playing bass is good for my health, and that it is a security blanket, but when push comes to shove there probably are other objects that are more mission-critical.
The next two levels are probably the true sweet spot of need concerning basses - social belonging and esteem. If you're on TB posting about basses, then you're fulfilling these two levels of Maslow's hierarchy.
The self-actualization level is where many aspire to, playing music to realize our inner goals and needs. Above that, I aspire to transcendence, but it is elusive. Thankfully it isn't that necessary according to Maslow.