Again, this is the standard agreement generally used in most real estate transactions. It is a formal proposal, usually drafted by your real estate agent or broker along with you. If the Seller agrees to all of the terms and conditions in the contract, they will simply sign the document. It then becomes a legally_binding contract between Buyer and Seller.
Corporations and many other businesses can use this type of letter as a precursor to a formal agreement. In it they can list the salient points of their intentions and what they intend to include in formal contractual agreement if both parties can come to an amicable agreement. Thus it can be used as a fall back document should any questions arise during the final contract negotiations. A good example of this might be a letter sent to a company's investor explaining their intentions to buyout another company and what it is expected to do to the value of their stocks.