We find exactly the opposite to be true. If you were to go to an unknown area, would you want a map of the area? Well that is exactly what the LOI is: a map of the major deal points. In today's busy world it is important to be sure two parties are going down the same road and in the same direction before spending too much time working out all the details of a transaction.
Note that the definitions are not too far apart, but the documents themselves are. The biggest difference is a Purchase Offer is a formal legal document, that once signed by both parties becomes a legal and binding agreement. If you are working with a real estate agent or broker, this is usually the document they will have you sign if you are interested in placing an offer on a property.