Playing complete works or single pieces
Ricardo Viñes, who gave the first performance of many of the Debussy works, was one pianist who began giving concerts presenting complete works, with a profile [Brody p. 53]. This was not that usual at the time, as it is today. Normally a concert would be a mix of very different pieces, often single movements. Viñes performed the complete pour le piano, the complete Estampes, the complete Images I and Images II when he premiered the works.
This was probably also something Debussy thought important. In 1890 the Fantasie for piano and orchestra was to be performed in one of the concerts of Société Nationale de Musique. D'Indy was the conductor, and he decided to perform only the first movement, since the program turned out to be too long. However, at the rehearsel Debussy quietly removed the scores from the musicians, so they ended up playing nothing! [Lesure & Nichols p. 30] He gave the reason for this stunt in a letter, addressed to D'Indy:
It seems to me that playing just the first movement of the Fantasie is not only dangerous but must inevitably give a false impression of the whole. On reflection, I would rather have a passable performance of all three movements than a fine performance of the first through your good offices. [Lesure & Nichols p. 30]
In another letter many years later (1907), he writes to his friend Gabriel Pierné, who conducted the orchestra works of Debussy several times, that he preferred a performance of La Mer in its entirety, and not single movements [Lesure & Nichols p. 185].
Debussy still often chose only a few pieces from his Préludes, to play in his concerts. A reason could be that he didn't want to attempt the more difficult ones in public. But also
Viñes did selections from the Préludes, and it could be that Debussy didn't think of the Préludes as a work that had to be performed in its entirety.