It is good to use an LOI to gauge the viability of purchasing the property based on the price an terms you are considering. IN other words, it is good to use to gauge whether or not the offer you have in mind is even going to come close to making sense to the seller.
Academic: In the academic world a letter of intent is known as a Statements of Purpose and is part of the application process. Real estate: Often times there are properties that are not listed on the MLS or with a realtor. It is at these times that a letter of intent can be used to let the owner of the house know that a potential buyer exists and thus the process of buying and selling the house may begin.