Many songs are published in versions in different keys to suit voices of different ranges. So, compared to a version for soprano or tenor, a version in a lower key will be sung by a contralto or bass, say. So not only will the music have a deeper, more resonant tone because all the notes are lower, the voice has the different tone quality of the lower voice-type.
If stringed instruments are playing, then keys such as G, D and A can sound brighter because there is more opportunity to use open strings. The composer might also take the opportunity to write chords that involve an open string, whereas they might be reluctant to write the corresponding double-stop if the music were in a different key, because double-stops are harder to play.
Interesting case in point: Mozart's Sinfonia Concertante for violin, viola and orchestra K364. Mozart wanted the two solo instruments to be equals, but the viola is not as bright as the violin. So he makes the viola-player tune the viola a minor second higher, to make it brighter, and he wrote it in E♭ major (not a bright key for strings), so that the viola would have the further advantage that 3 of its strings are now tuned to a♭, e♭ and b♭, which E♭ major suits just as much as D major suits a viola tuned normally.
Harp strings are also considered to sound brighter when sounding in their "flat" position than in their "natural" or "sharp" positions. (How much of a difference it really makes to the sound I'm not sure. A stopped note on a violin is stopped by a finger, which tends to dampen the vibration a little. But by contrast, a harp string is stopped by a peg on a disc which is part of the pedal mechanism.) At any rate, some music featuring the harp prominently is written in flat keys so that most of the strings can be in their "flat" position. Examples: Ravel's Introduction and Allegro for flute, clarinet, harp and strings in e♭ minor--G♭ major, and the harp interlude in Britten's Ceremony of Carols, in C♭ major.