The adjustable mainsheet bridle is used instead of a traveller. The adjustment, not surprisingly, is to get its length correct. If the strops are too long, then it might be difficult to get the boom on the centreline; if too short then centrelining is no problem, but too much leach tension might be applied, particularly in light winds. The kicker should be used to achieve the correct leach tension and the mainsheet adjusts the boom angle. Applying kicker varies the length of the strop as does the adjusting the mast rake, hence the need for adjustment. In practice it is rare to need to adjust the bridle once the rig has been set-up for the prevailing conditions of the day. Hope that this helps.