If a company database is synchronized daily via the Data API, why is it still recommended to use the quarterly exports?

If a database has been initially set up via the quarterly export, it can be kept up to date using the daily synchronization mechanism (as described here).

What the daily synchronization includes

The daily query checks for new publications from selected sources, such as:

• Commercial register and insolvency notices
• Annual financial statements (e.g. Federal Gazette)
• Manual updates (e.g. name changes)

As a result: 

• New companies being added
• Updates to existing companies (e.g. new financial figures)

What the daily query does NOT include

• Batch updates (e.g., contact data of German companies, supplied by our partner Echobot, are only updated once a quarter, directly before the date for the quarterly export).
• Algorithmic bug fixes, improvements, and additions (for example, new and improved methods of determining employee numbers)
• Structural additions (e.g., new key figures and new sources)

Finally, we recommend importing each quarterly export entirely into your database (and overwriting previously stored data).