With Set Rest Position, you can set where in world space your constrained object is positioned when its target weight is 0. Rest position gives you control of the constrained object, rather than relying on the last position of the target objects. Setting the rest position prevents constrained objects from sticking to undesired positions in the scene when their target weights are set to 0.
For example, a book is point constrained to a character’s hand. When the character places the book on the table, the book’s position on the table is set as its rest position. This prevents the book from popping back to the model’s hands when the weight of the hand changes to 0. The books target (the model’s hand) weight is then set to 0. The hand now no longer influences the book’s position. The character then proceeds to release the book, turn, and walk away from the table. The book does not snap back to the hand because its rest position was set.