Despite the wishes of many trainers throughout the years, Volbeat are a claustrophobic pokemon which flock to plains, fields, mountaintops, and other areas where their light can be seen by as many Illumise as possible. However, Volbeat were seen as a necessity for travelers for most of history, because they were the only pokemon whose light was both bright enough and safe enough (not being fire or lightning) to reliably light the caves which are the only safe routes between countless cities across the world.
The hard part for the trainer was always getting their Volbeat to actually light the way instead of remaining dark in protest at being in a cave. Some appealed to their pokemon's reason, pointing out that they would get through the caves faster, going so far as to venture through caves in darkness until their Volbeat relented to help them find their way out in either direction. The problem with this was that before the invention of the pokeball, and to a lesser extent today, for many simply refuse the ball and demand to travel outside with their trainers, there was little to prevent the Volbeat from simply refusing to enter the cave by flying away. Others chained their pokemon and scared or struck them to make use of the fact that when they retaliated, they did so by lighting up in the hopes of blinding their assailants; a few justly lost their eyesight for this scheme.
But ultimately, it would be science which solved the problem. The means by which they lit up was studied extensively until it could be taught to other pokemon, using an Ampharos' homologous tail in place of a Volbeat, and it was given the name Flash because a Volbeat's light is a flickering one.