Why do some websites add "Slugs" to the end of URLs?

I recently changed my website url format from:

www.mywebsite.com/index.asp?view=display&postid=100

To

www.mywebsite.com/this-is-the-title-of-the-post

and noticed that click through rates to article increased about 300% after the change. It certainly helps the user decide if what they're thinking of clicking on is relevant, in terms of SEO purposes though I have to say I've seen little impact after the change

Usability is one reason, if you receive that link in your e-mail, you know what to expect. SEO (search engine optimization) is another reason. Search engines such as google will rank your page higher for the keywords contained in the url

The slugs make the URL more user-friendly and you know what to expect when you click a link. Search engines such as Google, rank the pages higher if the searchword is in the URL.