Why is it important for every table to have a primary key?


So I have some tables in Sql Server that are essentially a list of sales, things like:

ProductID  SalesOrderID ProductFamilyID  ProductCost ProductSource 

and so on. In this case, none of the columns are necessarily unique, so I can’t create a primary key from any combination of them. In fact, the only constraint that I really have on the table is that I need every row in the table to be a unique combination of the columns. So I’m assuming something like a unique index would be the way to go there.

The only primary key I could add is something like an autoincrement primary key. But what would be the actual use of that, database wise? What are the possible problems with not creating a primary key for a table like this?