Fisher separation is fundamental to the theory of finance.
It was shown by Irving Fisher that given efficient capital markets, firms (in effect, profit making companies) should concentrate on maximising their NPV rather than taking into account the cash flows that investors (shareholders) need.
This is because an investor who needs cash can sell an investment, or part of it, and an investor to whom an investment pays more cash than needed can re-invest it.
This means that maximising investor's utility means maximising the NPV of their investments.
This simplifies investment decisions because we can value investments without regard to investors' need for cash. Therefore we can use valuation models such as CAPM and APT to make investment decisions.