There can be couple of reasons for that:
1) They actually consider him long term Buffon replacement.
2) They will rotate Buffon on his final year (Real rotated Casillas in 2014, Casillas played all CL games, but other keeper played La liga, Barcelona played Stegen in CL but Bravo in La liga). This is not really a thing i like but its become more popular in recent years, goalkeeper getting rotated between CL games and domestic competitions.