eLife is one of the few journals that pays peer reviewers.
There's a huge divide between different types of peer review right now. There's the typical peer review you get in a prestigious "traditional" journal, which includes a review of the scientific method and also the "importance" of the work (is it a new/original work, does it move the field forward, are the results groundbreaking, etc etc). Then you have what's becoming the norm for OA megajournals, which has been pioneered by PLoS One, which focuses exclusively on the methodology (ie is it good science, regardless of "importance"). To put it bluntly, that's an easier and cheaper way to review a paper. I'm not at all arguing it's the wrong way, there's a solid case that pre-publication peer review should be methodology-focused and we should let post-publication review of some sort sort out the "importance" factor of a paper later.
So in these two types of peer review, the "traditional" version that assesses for importance will likely lead to a higher chance of a higher impact factor for your journal, since you're assessing articles based on their impact on the field (theoretically leading to more citations in the long term). Couple that type of peer review with open access (like eLife is doing) and you get a really good formula for producing a high impact journal after only a year or two. But getting that type of peer review done, particularly if you're in a hurry, as you are when launching a brand new high-cost journal, is hard and expensive. So eLife has chosen to fund all the aspects of the process, including paying the reviewers. So that's one way of throwing money at the problem of going from zero to high impact in a short timeframe.