[Q] Do I have to invest the money BEFORE applying for the visa or status? If yes, why?

Because the law says so. Legally, the regulations state that the funds must be "irrevocably committed" to the investment before the visa or status may be granted. This requirement must be met by either showing that the investment has already been made or is actively in the process of being made. Also, funds can be considered to be irrevocably committed, however, if they are held in an escrow account solely contingent on the issuance of an E visa.

Practically, no consular officer or USCIS examiner would feel comfortable granting visa or status based on your words that you are going to make an investment. It's called "put your money where your mouth is" or "talk is cheap" law.