GHS says:
 
GHS Thin Core Phosphor Bronze acoustic guitar strings combine low tension, superior playability with balanced tone! Wound over a round core, these strings bring a warm tone to your guitar with and even tonality across the entire fretboard. A smaller core wire means lower tension on the soundboard, and easier playability for the artist. A must-have for vintage guitars that require lower string tension.
Why Should You Play Thin Core Phosphor Bronze?
You have a vintage acoustic guitar and are worried about string tension
You want a set of acoustic strings that are effortless to play and sound great
You regularly play extended sets and want a "comfy" set of strings