The Software Usage table's purpose was to hold non-discoverable items. What i think you are looking for, and what SCCM should be populating is Software Installations(cmdb_sam_sw_install). This one is that table that you will want to start looking into and understanding your data to move towards Counters and license compliance reporting. If you do not see this table make sure that you enable the Software Asset Management plugin after you enable the SCCM plugin as there are some changes made once that plugin is active to make sure data gets into the latest tables.