For privacy, as Duckduckgo doesn't track your searching behavior or anything.
And for avoiding the search bubble. Because, as a result of them not tracking your search history, they will not be able to create a search bubble the way Google does always do.
Now, what's the search bubble?
Well, I have a sheer interest in photography, and I often look up stuff about photography, like camera's, photocontests, ...
And you, you are totally not interested in photography, but you have bad eyes, and been looking at ophtalomoligists, and glasses etcetera.
When I go to Google now, and search for "lens", I will get only camera lenses in my resultlist.
When you go to to Google now, and search for that same "lens", you will get only eye-lenses.
While actually, you may be looking for information about the lenses on the hubble space telescope, and I might have been getting bad eyesight, and want to learn more about that.
With Duckduckgo, we will both receive the same list of results, with photographic camera lenses, telescopic space lenses and eye lenses, all cleanly listed.
And if that isn't enough reason for you yet: Duckduckgo is a nice little company with good intentions from Pensylvania. While Google is a big international giant trying to rule the world :-)