In accounting, the journal should has two ledger that is called double entry accounting. This method was introduce by Mediecci in 12th century at Italy. The father of accounting, Luca Paccioli is the first publisher of double entry accounting system.

It is useful to recognize the source and the using of money. For example, Your clerk buy a set of book from cash (assumption: ignore tax purchase such as VAT, GST, etc). You should record the transaction into expenditure account and cash account.

I cannot imagine accounting without double entry method. An officer could record a transaction twice time that will chaos the company bookkeeping. For example, the officer record expending cash and buying a book.The journal of the transaction will be wrong and certainly we cannot build a trial balance sheet.You will not get a correct financial statement.

An auditor must be confused without double accounting. They will confuse the source of the expenses and they could not audit the bookkeeping. 

As an accountant, you should record the journal into right account. There is at least six types of account i.e. asset, liabilities, equity, income, cogs, expense, etc. A transaction will influence two account. The accounting system build on two sides or ledger (sections) i.e debt and credit. Debit at the left column and credit at right column in ledger.

Each transaction should be record at debit or credit side at same value. Do not miss the entry or your bookkeeping will be fail. 

Double entry is only for note ledger book not day books.From note ledger, we can build a trial balance and then make financial statement.