That's just one reason. Some more:
A single search interface. The Firefox search box lets you pick multiple search engines, but then a different one is selected each time. For me, DDG is always selected.
I can change my search target without retyping my query. I type my searches into Firefox's search box and I always type the !bang part at the end of the search. If I don't like the results at Amazon, I can go change "!amazon" to "!newegg" and quickly re-search another site. I also frequently use this to redirect failed mathematical queries to Google or Wolfram Alpha just by going and appending a "!g" or "!wa" on the search.
As someone else mentioned, portability. I don't need to ever configure another search engine list as long as I live. My list on my work computer install of Firefox is always up-to-date with my mobile and with my home Chrome install. If I get a new computer or switch to a new browser, it's already ready to go.
It updates itself. If a new site comes along (or I discover a site new to me), I'll have the search ready to go long before I ever thought to search it (or configure it) myself.
newegg ATI HD4200
That said, I can perfectly understand why you don't want to waste the time to set them up if someone has already done it for you.Yes, I could spend my time adding custom searches to all of the browsers I use. Or I can just use DuckDuckGo and get most of the searches I want automatically and not spend any time at all.