This usually required when siebel table doesn't have any column that can support the required data type or when there is additional data required at the business layer.
Although it can be achieved by using the extension table, but it is recommended that fields that are shown on UI or used frequently in the script should exists in the base column.
This is because that extension column requires an additional join in the sql query. And every time BC based on the base table will be queried, extension table will also be called into the memory.
Here is one video from you tube that explain the whole process of extending column as a screen cast.
