Because he wanted to!
All of Chopin's music falls into the Romantic period of classical music. Music from that period tends to be very colorful, emotional, and expressionistic.
C major (no sharps or flats)Sharp Keys:G Major/E minor (1 sharp)D Major/B minor (2 sharps)A Major/F-sharp minor (3 sharps)E Major/C-sharp minor (4 sharps)B Major/G-sharp minor (5 sharps)F-sharp Major/D-sharp minor (6 sharps)C-sharp Major/A-sharp minor (7 sharps)Flat Keys:F Major/D minor (1 flat)B-flat Major/G minor (2 flats)E-flat Major/C minor (3 flats)A-flat Major/F minor (4 flats)D-flat Major/B-flat minor (5 flats)G-flat Major/E-flat minor (6 flats)C-flat Major/A-flat minor (7 flats)These are all the possible keys you can write in, enharmonic keys are italicised.
F Major has 1 flat in it's key signature. Which is the B flat
The mediant of B flat major is D.
the scale e flat major has a flat, b flat and e flat.
Nocturne op. 9 no. 2 (Nocturne in e-flat major) Minuet Waltz Fantasie Impromptu
I believe the song is Nocturne in E flat (Chopin)
Chopin, Nocturne E Flat Major Op. 9 No. 2
Generally, three of Chopin's most famous pieces include his Nocturne in E flat Major, his Prelude in D flat Major (also known as the Raindrop Prelude), and his Fantasie Impromptu. As far as "important" pieces go, Chopin has written a multitude of pieces that could be considered important, as he was a cornerstone in the Romantic era of piano music. Specific ones that come to mind are his Military Polonaise, his Ballade in g minor, and his Etude in E major (this piece is especially adventurous, even for the Romantic era. It was also Chopin's personal favorite melody).
Between 1830 and 1831, along with two others in the same opus No.9.
It is a popular nocturne written in D flat. It is mostly used in popular culture and wedding ceremonies. You can download the scores from sheet music websites such as IMSLP and SheetMusicFox.
Chopin - Nocturne, Op.9 No.1 in B Flat Minor.
Since I do not know the composer you have in mind, I will give the general answer as the Romantic period. Nocturne means "night music" or "night song," and came into use by the Romantic composers of absolute music (music for music's sake). Chopin probably is the most notable composer of Nocturnes.
Nocturne in E flat (Op. 9, No. 2 by Chopin?
The cast of Nocturne in E-Flat - 1914 includes: William Humphrey Rose Tapley
Nocturnes are compositions of a dreamy or pensive mood, especially it it was written for the piano.
Franz Schubert used a flat Major quite often. Frederick Chopin used A flat Major in several of his piano piano pieces. 23 three of them to be exact. He uses A Flat Major more than any other key.