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why people use "rain cats and dogs" to describe a hard rain

Nice to meet you sunny,

" It's raining cats and dogs" is an idiomatic expression. As you well said , it is used to drescribe how heavily is raining.

There are a lot of idioms in English Language, (a whole dictionary) Do you think we could share more here ?

thank you bigmama.I'm very interest in where some idiomatic phrasescome from.but i don't know where i can find them.i will be very grateful if you can tell me something about that thanks again bigmama

You can find the meaning of many English idioms here

http://idioms.thefreedictionary.com/

this site    http://www.phrases.org.uk/meanings/r.html      has origins for many, as well

here is their entry on "raining cats and dogs"

http://www.phrases.org.uk/meanings/raining%20cats%20and%20dogs.html

This one too in Ebaby

http://www.englishbaby.com/lessons/4633/member_submitted/body_idioms