Curious, why? You don't really need more than one browser on your phone. If you installed Chrome and haven't deleted it, one can assume it's because the person prefers Chrome. Otherwise you can just delete it after having tried it out. Or am I missing something?
It can't really even be for testing, since on iOS it's not like it's possible for Chrome to render any differently than in Safari!
My iPad has lots of rarely-used apps in it, as do most of my friends'. I haven't used Chrome enough to decide if I want to keep it or not, the mere fact that it exists shouldn't indicate preference.
Given the lack of Nitro support for non-Safari browsers, you'd likely need to use another browser to better support a site which was largely JS based.
Also, for some reason I can have many 10s of tabs open to pages on Safari (with a jailbreak tweak) whereas doing the same in a non-Safari browser causes a crash.
I use it on the rare occasion I want to move a url from the desktop (where I use chrome). Aside from that, Safari works well for me. (The desktop remote debugging coming in 6.0 is really nice, too.)
I'd probably use it for bookmark sync, too, but my bookmarks are such an unmanageable mess I usually just google stuff I need to find again. (Beyond a few very static bookmarks.)
It can't really even be for testing, since on iOS it's not like it's possible for Chrome to render any differently than in Safari!