The logic is when you made DDG your default search for the address bar. Then it becomes the zero-stop jump off point for all the other search engines they have !bang syntax for (which are thousands, I think).

I used to configure those as search keywords in Firefox (and before, Opera), which do roughly the same without the exclamation point. But on a new browser, even just configuring your favourite top 5 searches is a lot of hassle compared to just setting DDG as the default search and using their bangs.