Quick answer: as decoration. And of course never as a false trick to attract readers.
I'll just tell you something that happened to me today. I open the homepage of El País. Surprise! An image selling a graphic. Nice news, lately is not usual to see infographics at elpais.com.
Click on the article. The image is a blog about ecology, inside El País.
Ok, I need one more click to get into the post. I hope I can see the graphic in good resolution now. It's about why men contaminate more than women. Look interesting. I want to see the data.
Oops. The graphic is just a thumbnail, I have to click to see it bigger. I don't think that's a good idea, but I click.
Oh my god The graphic can't be zoomed! And it's in french! Anyway, you can't read the data. It's used just as a 'beautiful illustration'.... It promises information, but does ot give yu any data.
MI read the post, looking for an explanation. No reference to the data that should be displayed oin the graphic... Oh! yes. A reference. A link to other web. The blogger shows that he is not really interested in the graphic: "See illustration at the beginning of the post" writes. I'm getting mad...
Well, I click on the link, at least I hope I can see the data somewhere. The same size. I know now where the find the graphic... Anyway, I try to get into the article to see the data. But no. It's just for suscribers.
So, my idea is: the blogger doen't know what does the graphic show. Whic are the data. He just liked the image and took it. The frontpage editor also liked the image. He used it for the frontpage, no matter what it says.
I may say this is not usual at El País That's maybe the first time I see something like this on its web. I clciked three times, see for pages on its web. And now I'm mad at El País. Doesn't seem like a good idea...