Up to this point, a table has always been shown as a whole. When the table contained 1 million rows, it was possible to retrieve 1 million rows from it. If somebody had the rights to read a table, it was all about the entire table. In many cases, this is not enough. It is often desirable that a user is not allowed to see all the rows.
Consider the following real-world example, where an accountant is doing accounting work for many people. The table containing tax rates should really be visible to everybody, as everybody has to pay the same rates. However, when it comes to the actual transactions, the accountant might want to ensure that everybody is only allowed to see his or her own transactions. Person A should not be allowed to see person B's data. In addition to that, it might also make sense that the boss of a division is allowed to see all the data in...