Sometimes you need to log into a site to get to some walled-off data. Let's see an example.
Let's assume we want to crawl the articles on TheBrowser.com. TheBrowser is a manually curated subscription based website which recommends popular links around the web.
Once you have an account on the site you're wanting to crawl (TheBrowser's will cost you some $35 yearly but same procedure applies to any website with a paywall / log in area), log into it.