"Why not just a galaxy s3?" Because a Galaxy S3 is actually slightly bigger than the s4, thus for people looking for a smaller phone it wouldn't be a good choice. What I'm not sure about is how different this is going to be compared to the S3 mini.
[quotemsg=10886984,0,355427]Lol, this 'mini' phone has a larger screen than my Galaxy S advance. I wonder how many generations will it take till Galaxy S series is no different in size from Galaxy Note.[/quotemsg]
They won't go that far, Sammy intentionally engineered the S4 to be nearly the same size as the S3. And I think a trend to go a bit smaller will start soon. Especially considering more and more people have tablets everyday, and are slowly doing tasks they would do on there large smartphone and now just using there tablets.