That's a good question, one without a clear cut answer. The SGS 2 usually shown in pics was not available in the USA, and was one of the first Android devices to be sold worldwide and even then it didn't sell that well because we all saw the pitiful sales numbers. It wasn't until the SGS 3 in which Samsung went away from the look and feel of the iPhone (I'm not discounting the utility patents) that they really took off in sales which at that time a great many of the early Android adoptees were due for a upgrade and most chose the SGS 3.
I remember a friend who had a Galaxy that looked almost exactly like a 3GS, even with that "static row of icons", to reference KD. He even bought it because it was "pretty much like an iPhone, but available on T-Mobile".
In my opinion, Samsung initially gained separation from Moto and HTC and gained some name recognition from selling "pretty much like an iPhone" phones, complete with similar packaging and cables.
Once they gained traction (around the time of the Galaxy S I'd say), they then were able to capitalize and become a little more unique with the Galaxy S III.
I know Samsung has been big in phones for a long while, but I never remember them being a top dog in the premium phone market pre-Galaxy. Even with Android, I struggle to remember a successful Samsung Android phone before the Galaxy monicker. Pre-Galaxy, most people were touting the HTC Incredible and DROID.
So I'd argue that the Galaxy put them on the map in a big way, and the Galaxy was a 3GS lookalike. This have Samsung traction, and helped facilitate their rapid growth (which did come after they went more unique/large screens).
If Samsung is anything, they are smart. There's a reason they made sure even the small details were reminiscent (kind word) of Apple. They didn't do this just for kicks.