Well, because it's by far the best way to use Instagram on a non-apple device.
I love to browse pictures, write comments etc. at work, too - so, I guess using my private iPhone or iPad every second would be very flashy :D
The best thing is, that there is no limitation in comparison to the iPhone App besides the possibility to upload pictures, but as I'm photographing and editing every single pic on my iPhone, that doesn't count.
Keep up the good work guys - thanks