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. Although previously at Man City, his performance declined due to several injuries and also internal problems, but now, at Napoli, his role will certainly be crucial, with his ability, maybe it won't be as optimal as before, but it won't be bad. He is a professional player for at least the next 2 years, he is still very valuable and has high potential.
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.
The Premier League requires ability in terms of speed and often players who have entered an age like KDB have difficulty keeping up with young players. But it's different in Serie-A because skill can match speed because in Serie-A the style of play is slightly different from the Premier League.