Think of the number of emails you send and reply to in a given week/month/year. That's a whole lot of emails that could be telling folks about your site, blog, organisation etc. It only takes minutes to do and it's free! Try it. I've had links in mine for years (and it's definitely helped).
Below is a cool infographic about email signatures. It'll help you decide what to include in your own email signature. Hope it helps! :)
Click on image to enlarge: