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.