What is the purpose of archiving email? Is it saved somewhere? Where does it go?

I am working a job, that utilizes a tremendous ammount of email usage. At the end of my first month I would like to archive everything I sent and received in June to save room on my harddrive; however, I don't know what will actually happen. Will my old emails be cleared? Will I still have access to these documents? Will my computer slow down or speed up? Somebody please give me a detailed explanation. I appreciate it greatly.