Don't really understand the question, as you obviously know that a zip file is compressed, and clearly you don't need to compress it again to send it to another person.
Just in case you are serious:
A zipped file is a file or collection of files that have been compressed using a lossless technique (i.e. unlike say, an mp3 or a jpg file, it is possible to reverse the process and recover the file[s] with no loss of information).
How much the file is compressed will depend on the type of data it contains; a jpg or mp3 file will compress very little, and may even grow larger when zipped, but a Word document or a database file can often be compressed to as litlle as 20% of its original size.
As well as compression, zipping is also useful if you want to send someone (or back up) an entire directory structure with all its subdirectories and their contents.