Many mobile webpages, such as shown in this screen shot from Huffington Post, force the user to click on a button before displaying the whole article.
The clicking does not seem to improve the experience, nor is it intended to be used as a paywall to entice potential conversions.
It is unlikely to be for saving bandwidth, as many bandwidth intensive images load along with the button. It appears to me to be there solely as a source of annoyance.
Why, then, is this tactic so widely used?
Well, you can see the answer at the bottom of your screenshot.
Advertising
If you over-scroll you will see the ads. If you click 'read more' and then make it to the end of the article, you will see those ads again, increasing your likelihood of clicking.
Secondarily: this gives the site analytics about who is actually interested in the article beyond the first sentence and a half. Seeing as many sites are putting out click-bait you expect a lot of users to already feel cheated after reading the first sentence when they realise they are reading trash.
