You understand the concept of Refers? Everytime you connect to a site off a link, your browser sends that link to the next site right? follow that.
The referrer and agent part of Sim Spider just allows you to set that referrer and/or agent name that will be reported to the site you connect too.
You won't fool any cloakers into thinking you are a search engine spider by changing the agent name to a se spider. It is mainly a tool to strip the html and look at the 'naked' page with tags to see what a search engine will see.
The agent name is handy if a site is 'bigoted' and only allows certain (read: ie) browsers in the front door.
Referrer isn't all that important and probably should just be left blank.