You raise an amount that will get you to the next capital raise, and convince other entities that you are sufficiently capitalized (banks, lessors, contract counterparties, prospective employees), and deal with a moderate amount of contingency. You don't want to excessively raise, because it's excessively dilutive when capital is expensive. You also don't want to have to raise again with nothing to show for the spent cash.
Even if we grant your statement "there is no advantage of having the entire amount upfront"-- there would be a benefit to the investor of having option value about whether to continue to invest; option value that an employee has. It's easy to model this and see that option value is quite valuable. (Investors would really like it, so we do see things like efforts at tranched investments or leaving a round open... but entrepreneurs hardly want to have to sell an investor equity at the same price if the investor decides he wants it a year from now).