diff between $request->content($c) and $ua->post($url, {}, Content => $c)

Hi,

I'm wondering what the difference is between providing content to the 
HTTP::Request instance:

  my $req = HTTP::Request->new(POST => $self->url);
  $req->content($c);

and creating a LWP::UserAgent instance and using the Content pseudo header.

I have a more practical question next.