There is no filtering through legally written clauses and other such unnecessary information at this stage in the project. When the letter of intent spells out each detail clearly, the seller can come back with alternate terms of which he or she would be more accepting. This negotiation can go back and forth without the rewriting of lengthy pages that either party may misconstrue.
When "LOI" is used, it actually refers to the Letter of Indemnity. This is standard usage of the term and has been for decades. The Letter of Intent (hereafter referred to as LOI) is often used in specific domestic trades, such as in real estate or corporate mergers, but it has no standard usage in International Trade itself.