Firstly, divider rails are a practical matter. If your shutter panel has a height over approximately 72" with a width over 30", a divider rail is generally required to help support the integrity of the panels weight. These dimensions may vary slightly depending on the manufacturer and the material from which the panel is made.
Secondly, divider rails are a design feature which can add a unique look to your windows. Divider rails can be set to match horizontal mullion in your window or can be use to create a design look which splits the window in half or thirds. Design rules generally suggest splitting in half or thirds, but for some rules were made to be broken. As such, divider rails can be custom set at any location desired, assuming enough space is left for a couple louvers either above or below the rail. Also, multiple divider rails are also possible and are generally found on very tall windows. This application can be extremely stunning.
One item worth knowing - for windows with an odd number of louvers and a divider rail in the center you will generally see one additional louver on the bottom portion of the shutter panel. This is the most visually appealing look for windows with a center spaced divider rail.
Of course, we would love to help you decide whether divider rails are right for your plantation shutter project. Also, check out our online shutter cost calculator for a rough idea of what new shutters might cost.