Answer to Question 1
When you create a default document template in a library, all of the users start from the same template file which provides consistency to the document. For instance, if you create a travel reimbursement form and make it the default document template, it is readily available to everyone in the organization and consistent in the data recorded on it.
A second reason for creating a default document template in a library is that it enables people to store documents of the same type in the same place. Continuing with the travel reimbursement form, everyone who needs to access the form will find it in the library used to create it.
Answer to Question 2
A