This was the first classical piano piece in "Berceuse" (Lullaby)-form. Chopin introduced this idea into the piano repertoire. Many composer after him (e.g. Liszt, Karganoff, Grieg, Tchaikovsky etc.) were inspired by this idea and form, and created their own version/interpretation of it. They often used the same key D-flat-major, but not always. Grieg e.g. choose G-major, Tchaikovsky A-flat-major. Read more about Chopin's Berceuse: https://en.wikipedia.org/wiki/Berceuse_(Chopin)