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.
In Manchester City he has shown an extraordinary contribution by bringing Manchester City to win many trophies, now he has officially left City and joined Napoli. I think Napoli gets the best and better quality midfielder to boost their strength. But behind Kevin De Bruyne joining Napoli I think Lukaku played a role in persuading him to accept Napoli's offer because both players come from the same country and now they are on the same team.
On the other hand, Napoli is predicted to be much stronger compared to this season, especially since they managed to become champions last season, of course their enthusiasm next season will be much more fiery. Conte has succeeded in bringing Napoli back to being stronger and maybe will be more ready to defend the title next season.