OneSearch can search across many databases at once, but sometimes it is more useful to search only in one specific database. Searching in individual databases can reveal books, articles, and other sources that may not pop up in OneSearch. Individual databases will also have unique features, like limiting your search results to resources about a specific country, that OneSearch simply can't do.
These are large interdisciplinary databases with scholarship done primarily in English. You may, for example, be searching for articles on Japanese literature or a specific author, but could still see articles on the Japanese economy in your search results. Using the advanced search features of these databases will greatly improve your results.