Lighting controllers do for lights what mixing consoles do for sound. If you think of lights as performers, you can completely control the output of each at any time...and on many levels. Lighting controllers let you control color, motion, gobos, strobe, brightness and more.
The nice thing about lighting controllers these days is they can be a permanent installation or as mobile as a laptop or tablet. You can have a lighting engineer in the booth manning the controls or carry your lighting controller around with you making changes as you need them.
You can also preset scenes with lighting controllers, allowing you to easily control things that repeat consistently. That way, rather than controlling everything individually as it happens, you can set up your common and reoccuring lighting changes and have one button control it all.
All it all, lighting controllers have been one of the best dollar for dollar investments lighting engineers have at their disposal. And it allows them to spend more time with the creative side of lighting and less on the functional parts.
All prices are in USD.