A Mortgage Agreement is a pledge by a borrower that they will relinquish their claim to the property if they cannot pay their loan. Contrary to common belief, a Mortgage Agreement isn't the loan itself; it's a lien on the property. Property can be expensive and sometimes a lender wants more than just the loan agreement to back everything up. A Mortgage Agreement is the remedy in case the loan isn't repaid.
If you're in the process of buying property there's a good chance you'll need a Mortgage Agreement. Buying a home is often a person's biggest investment, and some collateral may need to be involved. As a borrower, you can't borrow an enormous sum of money without some incentive to pay back the loan - a Mortgage Agreement serves to guarantee the loan. Conversely, as a lender you're likely not going to want to lend a large sum of if you feel you won't get it back. A Mortgage Agreement puts a lien on the property and provides security to lenders.
Note: Depending on your circumstances you may need a Deed of Trust instead.