In the case of one who wrote an IOU document in his own handwriting, without witnesses, then the collection can only take place from unencumbered property -- even if that same IOU was later ratified in court. A bill of divorce is brought in the attempt to argue that such a document should function as a complete IOU and be able to collect from property that has a lien on it. Though the Gemara highlights the differences in the cases instead of accepting the argument. Also, in the case of needing the collect (and pay back) the guarantor... to avoid being strangled. With praise going to one sage, even though the halakhic decision is not according to him -- because the process of halakhic thinking is part of what is needed.