Why are they called Subsidiary Journals?

While the earliest recorded evidence of accounting goes back 7000 years to Mesopotamia - we can thank the Italian banking centers of Genoa, Florence and Venice in the 14th century for the version of double entry bookkeeping we use today.

In particular the Venetian model which had three major books of accounts, the memoriale (Italian: memorandum), the giornale (Journal) and the quaderno (ledger).  Which is still the underlying concept of modern day accounting. Of these we derive the manual bookkeeping methods of which you would have a collection of books or Subsidiary Journals and your General Ledger. 

• Sales Journal
• Purchases Journal
• Payroll Journal
• Cash and Bank Journal
• Asset Journal
• General Ledger

When using the manual bookkeeping methods, if you wanted to enter a sale - you would put an entry into the Sales Journal to record the sale and a corresponding entry into the Cash Journal to record the money received. Each of these subsidiary journals would then be balanced off at the completion of each month and the balances would then be transfered to the General Ledger from which you would then be able to produce your financial statements.

Today these "Journals" are simply modules in your accounting software. When entering a sale or purchases into your accounting software it will automatically record the double entry to the general ledger.  

While there is no need to manually close off each ledger at the end of each period - we still print off the trial balance and ensure all accounts including the Subsidiary Journals are reconciled and the books balance before reporting can be carried out.