tracingService vs Exception handling

Hi,

I'm looking at this code to learn plugins and programming, can you please explain to me what's happening in the catch block (lines 80-81 in the code sample at the link)?

catch (Exception ex)
                {
                    tracingService.Trace("FollowupPlugin: {0}", ex.ToString());
                    throw;
                }

What does the {0} inside the interpolated string stand for  / point to?
Why do I need the Throw instruction here (line 4 or line 81 in the code sample at the link)?
Where can I find the tracingService logs on Dynamics CRM online?