One of the things that's changing in this wonderful online digital world is the prevalence of video now. Time was when a photo or even more, simply well chosen words, made for a good blog post and thus created a following. This is becoming increasingly untenable as the readers (or the viewers) are demanding video.
And this is a problem for us one man bands of course. We can't go to Africa and create nature stock footage for example. Now, OK, this is a pretty minor problem for blogging if I'm to be accurate. But it is a big problems for the hundreds (if not thousands) of TV stations now available over the satellite and cable systems.
None of them have the resources to create their own nature stock footage, nor hd stock footage. Thus the rise of the agencies and you can click through any of those links to see what can be purchased, off the shelf, without having to send a camera crew anywhere at all. For someone has already done it all for you.