Why the new version, and why so soon? Mainly to provide late-breaking features that developers may want to adopt now -- but they come with a short-term (eight month) support cycle. For those who want or must use a version with a long-term (30 month) support cycle, version 6 is set for April 2016.
The Node.js Foundation has good reason to maintain a snappy pace for releases. An earlier strategy, where releases were more stately and less predictable, sparked criticism and dissent within the community -- to the point where a separate incarnation of Node.js, io.js, emerged in response. Io.js and Node.js have since reconciled, and timely releases of Node.js are now part of the overall plan.
One of the irritations developers face with the faster pace of Node.js development is making sure that native add-ons work properly across versions. Node's solution, the Node Abstraction Layer, is guaranteed to work across at least the two most recent long-term support versions of Node.
It's unlikely that the changes in Node 5 will break common applications, but any medium-size to large enterprise deployment of Node will want to stick with the current 4.2.x branch for maximum compatibility.