Instantiation of a class to initialize fields...

The most likely answer is that the Host property would contain code to split out the 'host'.

A possible alternative would be for the constructor to split out all components of the url and save them in individual fields. However, this would increase the memory footprint of the URL class considerably and so I think most library writers would prefer to split out these components as needed and take a small performance hit if they need to split them out more than once.