Sailmakers everywhere love using McLube SailkotePlus, below is a great summary from Ullman in the USA.
SAILKOTE Plus prevents mildew growth. Obviously the primary benefit to Sailkoting your cruising sails is the prevention of mildew. We probably dont need to mention mildew any more except to say that if SAILKOTE Plus is applied to your sails, you will have very little, IF ANY, mildew growth on your sails.
Furling and unfurling sails will be much easier. In addition to limiting mildew on furling sails your sails will operate with much less friction, and therefore will be easier to furl and unfurl. Sailkoted sails roll tighter. This is a real boon on systems such as roller masts/roller booms (where there is a limited cavity for the sail to fit in) the sail will fit better and not chafe as much when furling/unfurling. Also larger sails can be fitted or heavier more durable fabric used.
Cruising spinnakers will raise/lower more easily. A Sailkoted spinnaker/asymmetrical will be easier to raise and douse, especially when using a spinnaker sock, due to less friction. Often times when the nylon is wet, use of the spinnaker sleeve is made difficult by friction between the sleeve material and the nylon from the spinnaker (ever try to separate 2 pieces of wet glass?). If there is SAILKOTE Plus on at least one of the surfaces, friction is significantly reduced, creating an easier running spinnaker sleeve.
Key Benefits for Cruisers
SAILKOTE Plus prevents mildew growth. Nobody wants ugly mildew on their sails, whether you race OR cruise.
Less porosity and less water absorption on spinnakers. SAILKOTE Plus can decrease the porosity (less wind blows through the sail) of a nylon sail. This is particularly effective with older sails that you are trying to breathe new life into, as it will decrease the water absorption as well. Spinnakers that get wet during take-downs will dry much faster, keeping them lighter and faster.
Better spinnaker handling. Sailkoted spinnakers and asymmetricals are easier to raise and lower. In addition, asymmetrics are easier to gybe, due to less friction as the sail rides over itself during a gybe.
Genoas tack more easily. Sailkoted genoas will glide across the rigging and mast during tacks much more easily and sustain less spreader damage than un-koted sails. You should also consider spraying the lower rigging and mast as well. Telltales fly more freely, especially when sails are wet (or have you just been pinching all this time?).
Key Benefits for Racers
Everyone has an opinion, here are mine on sails and canvas, or whatever else I feel a need to rant about.