For his position at Chelsea, the most ideal is a striker, considering that Chelsea doesn't have a striker who is dangerous at the moment, choosing Nkunku is the right choice for Graham Potter.
Actually, Chelsea has a strong striker, Aubameyang. Unfortunately, Aubameyang seems hard to show his best in Chelsea, he experienced a difficult adaptation to Chelsea tactic. That's why Chelsea needs another quality striker, Chelsea needs a striker who fits with Chelsea game style. Having Nkunku in the squad will make Potter to have more options/alternatives in Chelsea attacking line. He can play with 2 strikers (Nkunku-Aubameyang), or choose Nkunku as a the striker if Aubameyang can't play effectively.
Every coach should have a fairly broad view like Graham Potter, he buys players who can fill in several positions and not just one position, so when the match feels difficult he can set a strategy by entering players who can fill these various positions and Nkunku is the best option so far, but I don't think chelsea will be satisfied with just one player, they will definitely buy more players in the future.