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.
Use a Letter of Intent if: You want to take serious steps towards negotiating the purchase of all or part of a company. You want to secure your interest in a company and request that the sellers exclude negotiating with another buyer.