Every time a batter step's into the box they should "measure up" the same distance to the plate. This will allow a hitter to have a consistent feel for the strike zone from one at-bat to the next. This type of discipline is instrumental for an athlete striving to experience a high level of success.
The typical way a hitter does this is by extending their outstretched bat to the outside part of the plate. This simple pre-pitch routine is so common place that it often becomes part of an athletes muscle memory - similar to a stride or follow through; however, it is essential that a batter completes this part of his regimen with purpose. Baseball is a game of inches and if a hitter doesn't prepare accordingly they are putting themselves at a major disadvantage.
A good point of reference for this are tee drills. When training on the tee it is extremely important to know exactly where the ball is located in the zone. This allows the hitter to adjust his swing accordingly and drive the ball to the appropriate field.
Once an athlete finds a consistent, comfortable and exact location in the batter's box, it is extremely important to always be standing in that same spot. Hitter's who do this successfully will have a better feel for the strike zone and find it easier to maintain proper mechanics. This allows hitters to capitalize on pitches they have been training for in practice.