I use LOI's a lot when I am trying to buy a property for much less than what the seller says they want, more flexible terms than the seller says they want OR if I am unfamiliar with the property and area I will use the LOI to start the negotiating process while I do my due diligence work in the background. Remember, most of the time an LOI is not a formal offer but mainly a point of understanding between you and the seller.
For the buyer to pursue financing. The financing process takes times and banks require either an LOI or purchase agreement as part of the loan package. So an accepted LOI is a critical component to securing financing.