Because there is a cellphone SKU for everyone, both in design and in direct quantity.
The latest big release, the Galaxy S4, arrived just about a month ago with its 5-inch 1080p screen and potentially dual quad-core processors depending on where you buy it. You could wait until late June and purchased on from the Google Play store containing the full Google experience. If that does not suit you, how about a 4.3" 960×540 version? That would be the Galaxy S4 mini, or at least one of the localized versions they will invariably make for multiple carriers.
Image, Samsung via Samsung Tomorrow.
Btw, why does Samsung watermark photos on their company blog? Anyone?
One of the constants between the computational hardware of each Galaxy S4 version is the 2GB of RAM; basically everything else differs between specific subversions of the flagship phone. Not the mini! For whatever reason, the S4 mini backs off on the RAM by half a gigabyte leaving it with 1.5 GB.
One of the main selling features of the large S4 is the eight-core (quad-core A15, quad-core A7) SoC developed by Samsung. It was available in the international version, the American version instead having a quad-core processor from Qualcomm. The mini, on the other hand, will contain a slightly lower-clocked dual-core processor.
In the other features: the battery is about 27% smaller albeit with less power-hungry components; the rear camera drops from 13 megapixels to 8 megapixels, whether or not that is worse picture quality is unknown; and the internal storage is 8GB (5 user-accessible), down from the minimum 16GB of the not-mini.
So beyond the name, there does not seem to be many similarities between the regular and the mini S4. It is basically software which links the two devices. The mini has access to services such as S Translator and S Health, although there does not seem to be any discussion of other services like S Travel and OCR software.
Samsung will officially unveil it, with hands-on demos to various press members, on June 20th in London.