When using cross-domain requests, it is impossible to read the response
data, and process it.
Using JSONP allows a client, from a specific domain (for example
/domain1), to query data in another doamin (/domain2), and analyze it,
using a callback.

Such a request would be followed by "callback=" parameter. For example:
http://foreman-server:3000/statistics?format=json&callback=analyzeStats

And in this example the response would be:
analyzeStats({"statistics":{"mem_totfree":0.0,"arch_count":{},
"env_count":{},"cpu_count":{},"mem_size":0,"swap_size":0,
"model_count":{},"mem_free":0,"swap_free":0,"klass_count":{},
"os_count":{},"mem_totsize":0.0}})

which will call the analyzeStats callback.
Whereas a regular request,
http://foreman-server:3000/statistics?format=json, will just contain the
JSON data.

More reading on JSONP:
JSONP - http://en.wikipedia.org/wiki/JSONP
JSONP Ruby impl -
www.simb.net/2012/02/06/ruby-and-jsonp
https://github.com/crohr/rack-jsonp/