VBA Setting Variables To Nothing Or Null

I've been looking at code a lot here and at the end I always see people ending their VBA code by setting the variables to nothing. Why is this done? Don't all the variables in the sub automatically get trashed when the sub ends?

So what would be the difference of me doing:

Code:
Sub emailUser(strSubject As String, strMessage As String)
'Allows you to email the user if an error occurs instead of giving a messagebox
'and stopping everything. This way if something can not be checked out, you know
'what happened and can redo it the next morning (or fix the error if need be)

Dim olApp As Object 'Outlook.Application
Dim Msg As Object 'Outlook.MailItem

[Code] .....

vs

Code:
Sub emailUser(strSubject As String, strMessage As String)
'Allows you to email the user if an error occurs instead of giving a messagebox
'and stopping everything. This way if something can not be checked out, you know
'what happened and can redo it the next morning (or fix the error if need be)

Dim olApp As Object 'Outlook.Application
Dim Msg As Object 'Outlook.MailItem

[Code] ........