There are two main reasons:
You can waste a lot of time and effort, and retrieve poor results, if you use the wrong tool, or use the right tool ineffectively!
Look at the "Help" or "Search tips" pages offered by the particular database or search engine to answer the following questions. Other sources of information are the description of resources on our Databases page, knowledgeable colleagues or your Subject Librarian. Another page covers how to use search engines, including Google, effectively and the limitations of Google.
Applying the above criteria will help you to decide whether a database or search engine is suitable for the search you need to make, and identify the best way to use it. However, you should also examine your search results carefully and see if they are what you were hoping for. If you are not happy with the results, don’t give up! A successful search will often take several tries. You can usually refine your searches by adding new terms which spring to mind once you have seen your initial results; or you may choose to limit your results if you feel you have retrieved too many.
Always remember to look at the "Help" or "Search tips" pages offered by the particular database or search engine you are currently using.
If you continue to find poor results, you should consider using a different database or search engine.