StoreSQLQuery is a user-defined variable and is mainly dedicated to debugging.
StoreSQLQuery is different from other context variables in the fact that its main purpose is to be used as parameter of the specific global variable called Query. It allows you to dynamically feed the global query variable.
The global variable Query, is available on the proposals list (Ctrl+Space bar) for some DB input components.
For further details on StoreSQLQuery settings, see Talend Open Studio Components Reference Guide, and in particular the scenarios of the tDBInput component.