This agreement is most often used in transactions involving a purchase. Sometimes two parties will know that they want to do a business deal together, but they aren't ready to sign an actual agreement. For example, they may agree that one party will sell his or her business for a certain price, but they do not yet agree on who will take on certain liabilities of the business. The parties can sign this document to show each other a good faith intention to work out a deal. It can also help parties get on the same page as to what they expect from the purchase.
While most are not legally binding, there may still be consequences for not complying with the agreed upon portions of the letter. A Letter of Intent is often seen like purchase or crowd-sourced orders, so they are often used to help secure funding or prove the value of the business. If the other party drops out or doesn't honor "in good faith" the letter, actual financial damage may be incurred to the company. In court, most often the judge will make decisions based on what implied intent is expressed in the letter. Sometimes there can be recourse if the parties do not follow through towards an actual legal agreement.