The LOI as it is now used is a misunderstood application of poorly informed brokers and traders. A more proper application based on established custom and practice in International trade, going back even to the old medieval "Law Merchant" (Lex Mercantilis) is: Buyer requests a quotation _ in modern usage this is an RFQ/Request For Quotation _ NOT an LOI. Seller may then reply with a Quote. Buyer confirms receipt of quoted prices, and requests an Offer.
In sum, the rationale underlying the Seller's demand for LOI, can essentially be summed up as follows: 1) That giving an LOI to a seller by a prospective buyer, is an indication that the buyer is "serious" and willing to purchase; 2) That use of the LOI is the usual way of initiating a purchasing proposal by a buyer, and is the right and proper way to go; and 3) That there is nothing for anyone in the deal to lose by a prospective buyer signing an LOI.