For example: Server Type, Content-Type, Connection status, X-Powered-By, Set-Cookie

The request has succeeded.
The information returned with the response is dependent on the method used in the request, for example: 
GET, POST, HEAD, TRACE

GET an entity corresponding to the requested resource is sent in the response;
HEAD the entity-header fields corresponding to the requested resource are sent in the response without any message-body;
POST an entity describing or containing the result of the action;
TRACE an entity containing the request message as received by the end server

so you will not be ignored by user agent

a client must be prepared to accept one or more 1xx status response prior to a regular response

proxies will be able forward 1xx response

more info:
www.w3.org/Protocols/rfc2616/rfc2616-sec10.html