If you are using an execution context that inherits from the NameRecorderMixin (nearly all do -- see chapter 8) then the execution context will raise a ApplicationError exception if multiple instances of a non-multi-select <al-select> tag with the same name are added to a form. The list attribute is used indicate that multiple instances are intentional.
The presence of the list attribute on an <al-select> tag makes the request merging in the NameRecorderMixin class place any browser request values for the field into a list (field not present is represented by the empty list).