The API decorators were initially introduced in Odoo 9.0 to support both old and new frameworks. As of Odoo 10.0, the old API is no longer supported, but some decorators, such as @api.model, are still being used.
When writing a new method, if you don't use any decorator, then the method is executed on a recordset. In such methods, self is a recordset that can refer to an arbitrary number of database records (this includes empty recordsets), and the code will often loop over the records in self to do something on each individual record.
The @api.model decorator is similar, but it's used on methods for which only the model is important, not the contents of the recordset, which is not acted upon by the method. So when you see api.model decorator set to method that's mean it operates on the model level, and not on the record set level. The concept is similar to Python's @classmethod decorator.
The api.model_create_multi : Decorate a method that takes a list of dictionaries and creates multiple records. The method may be called with either a single dict or a list of dicts:
record = model.create(vals) records = model.create([vals, ...])