Besides offering a different grip angle for the aft section of your longarm, a vertical fore-grip can be useful for another important task: activation switches for lights or lasers.
The TangoDown BattleGrip BGV-Mk46 vertical grip has a molded inset designed to hold standard pressure switches for SureFire lights. There's an included length of two-sided tape for securing the switch to the grip as well as two plastic zip ties for tethering wires to the grip to lessen the possibility of snagging or being pulled loose.
In addition to allowing ambidextrous addition of the pressure switch and being a stylish, comfortable grip option, there's another bonus: The inside of the grip is hollow and sealed with an 0-ring for weather-resistance. It's a great place to stash extra batteries, spare springs or other small items that might be needed at the range or in the field. Available color options are black, flat dark earth, and foliage green.