A tax preparer looks at the facts and tries to determine the appropriate reporting usually at the time of rushing through the tax season preparing multiple client tax returns.
A tax adviser looks at the tax theory that most benefits the client and then looks at the facts, regulation and case law history for evidence that supports or refutes the theory. The tax adviser is involved well in advance of the reporting period and helps set and document some of the facts and details that ultimately determine the tax treatment.
When the tax adviser is also the tax preparer, the client benefits from the familiarity with the transaction through all steps of the business process and ensures that the accounting and reporting is drafted in a way that maximizes the ultimate tax treatment as the best possible outcome.