Database Tuning Advisor Index Recommendations
How does DTA make an index recommendation? What is the technology behind that?


You can find the answers in a detailed paper on the Database Tuning Advisor published by Microsoft (click on the PDF link on that page to download the complete document).

The missing index DMVs are not a good substitute for a full DTA analysis. The DMVs show only very limited information opportunistically gathered during query optimization. DTA performs a much more thorough analysis, though you should not automatically create every index it suggests. You need to assess each recommendation on its merits.