Well, firstly, it does make the URL more readable. i.e. A URL such as
mysite.com/pianos is a lot more informative than a URL such as
mysite.com/?id=2347. To your common user, 2347 doesn't mean anything. It is random gibberish. The numerical ID-based URL doesn't give any indication about the type of content that is on the page. If a user decides to copy and paste a link to your page on another website, the slug-based URL will at least give them some sort of idea about the type of content that is being linked to.
Secondly, many users will share your content by copying and pasting the link, without much care for anchor text. In these cases, the link itself becomes the anchor text. If the slug contains keywords that are relevant to the page, and this link is "raw pasted" onto a forum, the keywords in said link will be interpreted by Google. This is evident by Webmaster Tools, which has a section called "How your content is linked." In this scenario,
http mysite com pianos, whereas the ID-based URL becomes
http mysite com id 2347.
Thirdly: There's CTR (Click Through Rate). If a user is searching for a specific keyword on Google and that keyword appears in the URL, are they more likely to click on that URL? From what I've read / experienced, the answer is yes.
Lastly, the title is there because:
- It shows up in the title bar of the browser.
- It shows up in bookmarks.
- Other websites can automatically grab said title.
- Google still uses the title tag to help determine the content of a page.
- It shows up in the history of a browser.