It's pretty simple when you look at the definitions of the two.
Accounting is the language of business used to display an entity's (person or company) financial status through various financial statements and analysis reports.
Auditing is the evaluation of the above mentioned statements and reports in order to ensure that they are fair and no overstatements have been made.
Therefore, accounting is necessary because it is the only way to exhibit the financial status of a person/company... which is important when making business decisions (taxes, sales, loans, etc.)
On the other hand, auditing is a "luxury" because it is only a "double-check" of sorts, making sure that the accounting has been done correctly and fairly. That's why only a few select people/companies will be audited each year... it is not necessary and would consume way too much time and money if EVERYONE were to be audited.
Ex. Fred owns a major company. An accountant goes through his records and creates various financial statements and reports exhibiting his worth, his assets, his profitability, etc. (this is the process of accounting) Then, these statements and reports are submitted to banks, referred to in tax reports, handed out to shareholders, etc. SOMETIMES, after these decisions have been made, an audit may be issued. If so, an auditor would contact Fred and ask for the records that his accountant used and ensure that his records do not contradict what he reported on his statements and reports.
As you can see from the example, auditing is NOT necessary nor required... it's simply a way of making sure that people/companies are not lying on their financial statements.
