Answer 1

take an example:

you have a shop's database with the tables Client and Order. the columns are ID and Name for Client and ID_Order, ID_Client, Product and Value for Order.
You want copy these tables to a data warehouse and need a primary key. ID of Client is not good for the warehouse because If it changes on transacional you must reprocess all tables on warehouse, so you put this column only on Client and create a surrogate key for the Client. Finally you create the Order table on warehouse with the surrogate key, id_order, Product and Value.
With this model any changes on Client Will be updated only on Client table.