Suppose you have a website about a subject, for instance, popcorn.
Everything in your site will be about popcorn and your regular visitors are popcorn enthusiasts who will understand that even articles without the keyword "popcorn" in the title are about popcorn.
How do you tell this to Google, if meta tags are not being used any more? Or do you have to keep adding "popcorn" to titles to have your articles show up in searches and news alerts? It's very awkward to keep adding a word to titles for this reason.
For instance, your title is
Why corn is good for you, but it won't show up in Google searches, so do you really have to change it to "Why corn, from which popcorn is made, is good for you"?
I've noticed that adding the keyword to the website name, as in
Why corn is good for you | Popcorn Every Day is not good enough.