|It improves your effectiveness with ranged weaponry.|
Slaying bonuses are always desirable, but not all characters wind up using ranged weapons. If ranged weapons are your primary form of attack, these are an extremely welcome find. Spellcasters would likely prefer gloves of strength or dexterity (or a nice artefact), while melee fighters might find them unnecessary.
Prior to 0.15, archery gave +5 to accuracy and +3 to damage, at the cost of -1 slaying in melee.