Essentially each country who has agreed to send such spacecraft wants to build their own spacecraft. This is most noted in US/ Russian, but is to a lesser extend to European/ Japanese partners. Breaking them down by country, we have the following:
- United States
Okay, so there are 4 parties who have build spacecraft. Why did each of these build their own? Mostly, they want to build them as both a matter of national pride, and a means to develop new technology. This development of man rated technology allows each of them to enhance their space capabilities, and generally speaking is good for the economy of the affected partners. There are two that have developed more than one, let me explain those.
- United States- The Shuttle had to be discontinued, and a replacement was needed. Two were provided for cargo replacement, as a means of redundancy, which allows for the United States to continue to send spacecraft in the event of a problem with one.
- Russia- The Progress/ Soyuz have both been around for a long time. One is manned, the other unmanned.
So, basically it's a chance to build technology in one's own nation (Or EU), and the pride and technological advancement that comes from such. In addition, valuable redundancy is gained from such.