On Offer acceptance there is either a contract generated or a pro forma invoice generated. Since the offer is binding once accepted, the offer if well enough detailed and advised can and often does take the place of the contract. Customarily in Real Estate transactions the purchase offer itself is generally also the contract for purchase as well. The offer becomes a binding contract once accepted.
LOI is used as manipulation tool at the hands of unscrupulous sellers & agents. Often times, obscure or scam_oriented persons who claim to be crude Sellers, or represent themselves as sellers' agents, mandates or brokers largely by an Internet contact or communication, employ the LOI merely as a tool to quickly "corner and box in" a prospective buyer to a purchase deal, before the prospective buyer may demand that they provide their business profile or show him something tangible to demonstrate that they are truly legitimate sellers. Such sellers would persistently demand that the prospective buyers hurry and issue them an LOI right upfront purportedly as proof that they are "serious" about making the purchase _ that is, before the buyer may probably start raising some probing questions about them or their credentials as legitimate sellers.