When you need something relatively simple and common. Keyword searching, Northern Light groups results into 'Folders' of related topics. Alta Vista is a particularly good search engine to use for locating images and video clips. When you need something relatively simple and common. When you want to narrow your search and look at specific area.
Users are generally better off using another search engine for the obscure and hard to find.
Multiple options, including phrase matching and keyword searching. Attempts to establish relevancy by looking for keywords in titles of websites. When you want a limited number of hits with a reasonable expectation of finding what you need or when you are looking for content in .pdf files. No other search engine covers .pdf files. When you want a limited number of hits with a reasonable expectation of finding what you need.