Not fulfilling the potential itself, but rather the capacity to do so, (which can only properly be measured by the actualization / acting-upon-of said capacity). As to why -- well, fundamentally it's the notion that maximized instrumentality is the maximally optimal instrumental state. From there the question becomes; "is maximized instrumentality useful?"

That is a "self-proving" terminal value. One need only ask the question to see that it implies its answer. "Is being useful useful?" Well... yes. Whatever it is you want to do or achieve is transparent / irrelevant to this.

Being useful is useful. The "use" of being useful is that it's useful.

These are essentially tautological statements.

So when I ask, "what's the use of being happy" -- saying "It makes you happy" is true (tautologically) but not an expression of utility, whereas having utility is useful because it's useful is also tautologically true but is an expression of utility.