As Retro says ^^^
The catwalk shows essentially showcase the trends that the whole clothing industry will adopt over the next half year.
It's a way of all the designers saying ... Look, don't spy on me, because I'm putting it all on display.
Without Fashion Week, the clothing industry would have no general direction. Manufacturers who failed to guess the trend would face disaster and job losses. When the media say that "style x will be the in thing this year", it's because the trend appeared on the catwalk, and all the high street shops are ready to follow.
Without this system, a lot of high street clothes retailers would go out of business, and high street prices would have to rise because of the market risk.
So, bottom line ... the catwalk fashion shows enable the mass market to continue wearing "fashionable" clothes year after year.