It really shines for me, since I have pentadactyl installed in my firefox. From the blank tab page, I can be searching github (or the python docs, or <insert one of the multitude of !searches here) with the following keystrokes:

  o !github language:python

'o' tells pentadactyl to go to a url. When I don't give it a URL it instead sends what I type to my default search engine.

The funny thing is that unless I am actually searching DDG, I almost never see it.

Sure you could do this by defining a whole bunch of search aliases, but it is more fun to think, "I wonder if I can search the Arch Wiki from here," type `o !arch some search`, and all of a sudden find yourself on the arch wiki search page.