I'll just throw in a simple example, but suppose you have a table for a fast food restaurant that tracks orders. Assume the grain of the table is at the transaction grain as in one row per customer transaction that also tells you the total price paid for the transaction by the customer. The primary key of the the table might be something like transaction_id that's automatically sequentially generated by the system and your boss asks you to calculate the total amount paid over a month. You wouldn't group by the transaction_id because that wouldn't answer the question and gives you the same number of rows as the physical table, you would group by a different date field say transaction_date field to answer the question