This function is available thru item DataBase inside File menu. It is used to the possibility of saving results of decompilation into separate file. It would usefull for more faster loading of results of decompilation in future.
If you need turn back to analysis of the program or bring these results to your colleague then you don't need decompile the program again. By using full decompilation (when feature fast decompilation is turned off) this process may last a lot of time. More usefull is to save results of previous decompilation into database.
VB Decompiler uses internal packed format of database. It allows to make the size of database more optimised. Databases are backward compatible. It provides the possibility of store information without a risk of impossibility of open old database in new version of VB Decompiler.