It was a round designed for a Personal Defense Weapon (PDW), which is more than a sidearm and less than a full rifle, for troops that couldn’t always carry an assault rifle.
Specifically, it was designed to replace pistol-caliber submachine guns as PDWs (like the MP-5K and such).
FN came out with the 5.7x28 round for both the FiveseveN pistol and the P90 PDW (sorta like a submachine gun). HK folowed up with a similar round for its MP-9 PDW.
The intent was to give greater penetration for the weapon (lighter, faster round with armor piercing rounds) because more and more troops and security were encountering body armor on the battlefield.
As soon as the FiveseveN pistol hit America, anti-gun sissies were crying about the new “assault pistol” with armor piercing capabilities. FN relented and restricted the good SS190 ammo and offered a lead-core round to the civvy market.
See 68 about the FN P-90.