Each database has its own method of organization, its own strengths and weaknesses in coverage, and its own unique search engine.  The obvious point is that you cannot get to any of the databases without choosing a search engine first, and if you choose the wrong search engine, you definitely will not find what you are looking for.  Even if you choose the right search engine, if you do not tell it to search the right databases, you also will not find what you are looking for.  Do the paper periodical stacks look any better to you now?