Another lesser known reason to use the traveller; it reduces the need to ease the main sheet. This is specific to boats with no backstay (most cats and some newer monos). My cat, for example, has no backstay, allowing for a lot of mainsail roach, but compromising forestay tension. All beach cat racers should know this. While you may consider it a racer's subtlty, it is a signifigant factor, and we are all racers when going to windward, if we are any kind of sailor. OFf the wind we kick back!
(the full explanation)
Sail Delmarva: Driving to Windward...
"No Back Stay. This means that the forestay cannot be kept tight unless you want to turn your boat into a banana and over stress the shrouds. Although swept back, they are only designed for the side force and a trace of forward pull. Real tension on the forestay comes from mainsheet tension.
Why must the forestay stay tight? Sag allows the genoa to become more full, since a sagging forestay has the effect of injecting more sailcloth into the sail. The draft moves aft, the slot is pinched, drag increases, and lift does not. This is OK off the wind, but not to windward, since heeling and leeway (sideslip) increase, which also increases drag. Going to windward is about lift:drag, not just power.
How do you keep from easing the mainsheet in strong winds? Ease the traveler a little, being certain to keep the main outhaul tight (a full main pinches the slot). Reef; it's better to keep a smaller sail tight than a larger sail loose. You will see monos with the main twisted off in a blow. Ignor them, they are not cats. Use the traveler instead. It is also physically much easier to play the traveler than the main sheet."