-- the universe won't wait for you
"There is no significant meaning behind Ho-Oh. It is mainly a plot device to show that Ash is special. Some believe it is used to symbolize the start of a new adventure; seen when Ash started his journey, and again when he left to Hoenn with Pikachu alone." [x]
"Ash first saw Ho-Oh in Pokémon - I Choose You!. It appeared on his way to Viridian City during his first day as a Trainer. He was lying on the ground with his injured Pikachu and saw it fly over the rainbow that appeared in the sky after a storm. His Pokédex could not identify it and explained that many Pokémon were still unknown. In Pokémon Emergency!, he tried to explain to Professor Oak what he had seen, though the image he used to point it out appeared to be of an Articuno rather than Ho-Oh. Either way, Professor Oak didn't believe him and brushed it off as Ash being mistaken. This gold colored version of Ho-Oh was an early concept of Ho-Oh as a Pokémon, like all other Generation II Pokémon at that time had not been fully developed yet." [x]
Basically they just thought it'd be a cool thing.