The Company prepares the accounts ("subject matter").
Auditors judge and give an opinion on these accounts based on some criteria (e.g. IFRS).
They all do it for users.
Users get more confidence, because the auditors express the opinion that the accounts give a true and fair view.
Therefore they are taking less risks.
The company needs a contract ("Engagement letter") with the auditor.
We need to make a report for users.
We need a method (getting evidence about the company).