Add a detailed description of mortgage-backed securities in the context of the Global Financial Crisis, which includes:

1. A detailed and specific outline of mortgage-backed securities themselves,
2. an explanation of why they can be useful financial instruments,
3. an explanation of the role they played in the Global Financial Crisis, and
4. a comparison between 2) and 3), explaining why the potential benefits and uses of mortgage-backed securities did not manifest in the Global Financial Crisis.