They see his achievements, see KDB's ability as one of the smart players especially on the field, as a creative and smart playmaker to open up opportunities.
KDB's presence in the Napoli squad will certainly provide additional strength and we know he is one of the creative players who can send measured passes to attackers and even at Man City himself he often transforms into a goal scorer when the attacker gets tight guard from the opposing defender. His performance has indeed declined after recovering from injury and maybe he can reach a good level at Napoli because in Serie-A speed is not the only reason but intelligence is very much needed.
With the arrival of KDB, at least, Napoli's strength can be further increased, as an Attacking midfielder, he not only focuses on increasing the club's attack power but he also has intelligence as a playmaker that will be very useful for Napoli. Moreover, for now, Napoli's current attack is only Lukaku who is indeed prominent. KDb's performance will not be completely back to what it was before he was injured, but if he can be optimal, he can still strengthen Napoli quite a lot, what is important is that his injury does not recur.