If you carefully look at the bottom left it tells you.
The term INBOX is derived from the office language IN-TRAY.
Would you use your IN-TRAY as your filing cabinet? Hopefully not :)
Why Bother clearing your INBOX?
All of your Emails, Folders, Attachments, Contacts, Calendar and Tasks are stored in one PST or OST file. To keep your Outlook happy and healthy it is important to keep this file from getting too large. Every time you open Outlook the first folder it opens is your INBOX - ensuring this only has a handful of emails in it means it runs faster and can significantly help your email not to lock, crash or at worst corrupt your PST file. (If it gets corrupted the worst case scenario is that you would lose everything)
Here is a quick tip I use for paper work which could also be applied to your INBOX.
Urgent = High, Important = High
If the email is Urgent and Important (for your role) DO IT – Deal with it right away
Urgent = Low, Important = High
If the email is not Urgent but Important (for your role) FILE IT – File it in folder (either a folder that’s appropriate for that matter or just create one folder called To Do)
Urgent = High, Important = Low
If the email is Urgent but not Important (for your role) FWD IT – Forward the email to someone else (i.e. delegate the matter to somebody else) Then delete the email
Urgent = Low, Important = Low
If the email is not Urgent and definitely not Important (e.g. SPAM) - BIN IT – Delete the email immediately. If you’re really not sure you could archive it.
Delete your Deleted Items
Your Deleted Items folder also contributes to the size of you Outlook File. If you right click and select Empty Folder you may receive a warning that you are about to permanently delete your deleted items - if you are sure proceed - this will help reduce the size of your mailbox.
Archive your Email
As previoulsy stated - in Outlook all of your emails, email attachments, folders, contacts, calendar and tasks are stored in one file. Setting up an Archive ensures that old emails you still wish to keep are moved from the live file into a separate second one (usually called archive.pst) Unfortunately this is not available on a Outlook for Mac.