Using references you create a reference to another table, without adding any constraints, or associations. Which means that is just a way of signaling the database that this table has a reference to another.
Creating an association will add a foreign key constraint to the attributes. And the you can perform all the magic behind it the association functions. i.e User.getTasks();
More info about references here: https://sequelize.readthedocs.io/en/latest/docs/associations/#foreign-keys_1
About association here: http://docs.sequelizejs.com/manual/tutorial/associations.html