Why didn't he just notate it with treble clef from the start?
The grand staff is the definition for keyboard sheet music.
As the title of opus 11 is 24 Preludes for Piano the composer could have notated from the beginning the treble clef.
The two clefs are emphasizing that there is a change of clef.
It might also have been that he had bought empty sheet music paper with prepared grand staffs but it doesn’t seem: