They hold up the shrouds that hold up the mast
By Don Launer
Spreaders are struts attached to the sides of a mast to hold the shrouds away from the mast and increase the angle at which they meet the mast. The greater the angle between shroud and mast, the lower the shroud tension required to provide lateral support and, therefore, the less the compression on the mast. This means the shrouds’ diameter and the mast section can be smaller, thereby reducing windage and weight aloft.
A boat’s beam limits the length of the spreaders, so boats with very tall masts usually have more than one set of spreaders to achieve the desired angle of attachment at the masthead. Another reason to use multiple spreaders is to allow the shrouds to be mounted inboard to permit closer sheeting angles for headsails.
Very often, the short stout mast on a gaff-rigged boat will not need spreaders, but most high-aspect-ratio Bermudan-rigged boats will need at least one pair.
Originally, spreaders were called crosstrees, but that term is not used on modern recreational sailboats.