Why? Because during the Renaissance, sleeves themselves became an important fashion focus, too glamorous to be kept under wraps. Men and women both wore gowns with plain sleeves that they covered with detachable, elaborate dress sleeves - pleated, sewn like wings to shoulders, adorned with slashes and/or studded with jewels.
Portrait of a young girl, 1540's - Venice, Italy
Mary Tudor and Charles Brandon
Favorite styles were the finistrella, two or three panels attached at intervals, to let puffs of the underchemise show through.
The wicked virago, a full, bloused sleeve reined in at various points with bands.
Or the lavish puff of every conceivable size and design.
Eleonora Gonzaga, 1538
One dress and a dozen sleeves could make any number of different outfits.
Source: Let There Be Clothes by Lynn Schnurnberger