Example:

$ svn copy foo.txt bar.txt  
A    bar.txt

• When would you use this technique, and why?
• Will this command (taken from svn's "red book") creates a copy of <foo.txt> while preserving the history of it to be shared with <bar.txt>?
• If I'm changing <bar.txt>, what will happen to <foo.txt>?

What are the equivalents to this in other modern systems (Clearcase, Accurev, Perforce)?

Clarification:
Let me emphasize the point I'm searching for:
Is this kind of branching out on a file level?
What happens if you use it in the same branch, i.e. create a copy of a file and than start changing that new file. all in the same branch?
I understand that it is also used for tagging but what is interesting me is what to expect when performing <svn copy> On the file level