Genres are relevent to audiences becuase  it helps them choose a film when they go to the cinema or pick a dvd. I love horror films so if I know I am seeing a horror film then I will have an expectation which is to get scared. If I get scared then that expectation has been suceeded therefore I come away happy. Because I know its  a horror film it means I will look for things like something jumping out of you. If I did not know it was a horror film then I would not be waitng for anything therefore I would not get the whole satisfaction.

Genres and production company- Genres help production companies find the right cast for a movie. If a musical was being cast the production company will probably look at other musicals and maybe select a well known musical actor for the role. Some actors will be cast in certain genres like Jim Carrey normally stars in comedies. Because he is known for comedies it means production companies who may be looking for a lead to star in a comedy can cast him without an audition because they know he is funny because they have seen him in other comedies and know that is his specialty.
Equally some directors stick to the same genre which means production companies know which directors are best suited therefore when they come to get a director they can rule off any directors that don't direct the movie genre that they are casting for.

Distribution- Targeting a film is a huge part of a film. If a comedy is coming out then they will want to push the fact that it is a comedy in the trailer or poster so that comedy lovers will come and watch it. If horror film trailer does not show the fact that this is a horror film then horror fans will not see the movie. Equally someone might go and see that film but not be aware it is a horror and be disappointed because they thought it would be something different. If distribution companies understand more clearly who they are selling to (because there is an existing fan base for that genre) then it is easier for them to target the film.