
A myth is most times a symbolic representation of reality, and it's intended to be a guide that helps us to deal with life's challenges.
In first place, myths have been used by humans since oral tradition exists, long time before writing was invented. In its primitive origins, myths used to speak about forces of nature, then evolved to stories about gods and heroes. The purpose of those stories was people learned some universal true deducted through observation of human behavior or constants in how Nature works. Those stories was made in a format of kind of condensed and exaggerated study case where the characters make decision and we can be witness of how those decisions worked in the context of the story, thus we be able to compare the results with the goals the characters tried to achieve.
In my opinion, it's more enjoyable to listen this kind of story than simply listen the conclusion, but in addition most myths provide a context and some sort of circumstances that might be extrapolated and compared with real life situations.
However, myths are far from being just a simple collection of success stories supporting a methodology related with successful living. In first place, myths are encoded in order to reach our unconscious. Actually, those stories shows astonishing tragedies or inspiring epic journeys (sometimes they mix both) that are very unlikely to be faced by a regular human being in his/her lifetime. However this sort of exaggeration is useful to highlight the most relevant attributes of characters and situations in order to be processed not by our conscious but by our unconscious. It seems to be easier for the unconscious to process symbolic representations of reality than the real thing. This is important because myths are made to warn us about danger or to bring us hope, and if unconscious is aware of these messages then I guess our response to life's challenges would be the desired one in a more natural or automatic fashion than when a similar situation is just remembered by the conscious.
On the other hand, if myths weren't important then they would be forgotten long time ago. Indeed, nowadays most of old myths like Noah's Ark, Gilgamesh's journey, Ulysses' adventures , King Oedipus' tragedy or Romeo and Juliet's story still being popular. In addition, year after year people create new myths, which is the case of The Lord of the Rings, Star Wars, The Matrix, X-Men, Superman and a long etcetera. All those Twentieth Century's myths are in part so seductive because they follows the same encoding than ancient myths. And they worked as well as old myths still doing.
In conclusion, myths are important for people because they enlighten us about how to deal with life in an unconscious level, and their effectiveness is proven by ancient myths which still working as well as the new ones.