What is the most important reason for citing a source when quoting it directly?

So you don't plagiarize, because that's illegal. Also, so whoever's reading knows where you got it from so they can go there and read it themselves.

according to greek mythology, persephone, the queen of the underworld, was the daughter of zeus and demeter, the goddess of harvest and fertility. she was also called kore, which means "maiden" and grew up to be a lovely girl attracting the attention of many gods.

explanation:

you can talk about what you learned about them. and you can talk about what happened on you're journeys with them. and then you can put all of the thing you learned about them in you're diary.