BU_ID is primarily created , to provide a setup to have multiple organizations in the same application. This concept will be very useful when people store the information about 'sister' concern which always say.

At this moment, it is important to differentiate the different critical business entities across organizations, such as ACCOUNT, PRODUCT, POSITION and so on which are in the top hierarchy. That is why to identify an unique account across organization, BU_ID has been added into the user key of most of the table and thus inserting these data into siebel through EIM/Regular import requires BU_ID to get populated.