"Whereas the late King James the Second by the Assistance of diverse evill Councellors Judges and Ministers imployed by him did endeavour to subvert and extirpate the Protestant Religion and the Lawes and Liberties of this Kingdome ... the Subjects which are Protestants may have Arms for their Defence suitable to their Conditions and as allowed by Law."
– Bill of Rights 1689
It is an ancient and established right of Englishmen to bear arms in defense of life, property, King and Country. Firearms allows you the means to exercise that right.
Damage done is % damage, weighted to be near 35% for Muskets and near 15% for Pistols.
Guns no longer do % based damage to animals. [Patch 1.2]
There's also a drain to Black Bile to concentrate and aim, about 20% for Muskets and 10% for Pistols.
Guns also Snare your opponents, Pistols is less effective than Muskets. But pistols can be dual wielded and are slightly faster than Muskets.