Another huge reason is to prove concept and make sure you're not fooling yourself. If you can get others to put in large amounts, it's an (imperfect) indicator

Another huge reason is to prove concept and make sure you're not fooling yourself. If you can get others to put in large amounts, it's an (imperfect) indicator of a kind of traction. Musk probably doesn't feel like he needs that validation, so the main reasons to take outside money would be leverage and diversification. The downside is some loss of control.