Investors follow investors. It will be much easier to get the next investor on board, once they see somebody else already put money.
In addition to that, when you talk to investors, they pick up on your self confidence signaling. If you already have an investor, you will talk differently and your body language will be different. They will pick up on that. Nobody wants to be the only person that puts money in a project, if they go down, they will feel better going down with someone else.