Harmful voltage spikes caused by common control solenoids switching off can cause permanent damage to sensitive electronic devices ( such as injector drivers). The solenoids in question include, but are not limited to Trans-brake solenoids, line-lock solenoids and nitrous solenoids.
By adding a TVS diode across the solenoid you eliminate harmful voltage spikes produced by the solenoids. Besides protecting your ECU from voltage spikes, an additional bonus of the TVS Diode is that it allows the solenoid to turn off faster which can be advantageous, especially in a drag race application.