Why Tables command is required

This statement creates a structure with the same data type and the same name as a database table, a view, or a structure from the ABAP Dictionary. Particularly used in select options. 

Whenever you use Select options, the structure of the table should be known by ABAP. 

For this purpose Tables command is required. 

Ex: SELECT-OPTIONS p_lland FOR vbco7-lland.

The structure of vbco7 should be known to that report/program.

So we use , TABLES : vbco7.

For this purpose tables command is required.