I think that work selected and analyzed by peers allows a greater expansion of the dialogue involved in the topic area. The idea of work that has been incorporated into peer reviewed journals and assessed can allow the reader to understand how different thinkers view the particular topic. It can also bring to light a greater understanding of the topic area in general when seen in a broader context. That being said, I do think that there is some value is examining a topic outside of the realm of peer reviewed journals, which usually select the Status Quo. There are many examples of ideas that were rejected by peer reviewed journals at the time, but are now seen as basic elements of the discussion. For example, mental illness is something that is understood in greater quantities now than it was twenty or thirty years ago. Peer reviewed journals spoke much differently then of mental illness and its treatment and how it should be addressed as opposed to now. If one relied only on peer reviewed journals as the sole beacons of truth, then the advances in the field since would have faced even more inertia than they experienced.
Agreed with the above posts. To take the idea further, think about crucial research fields such as medicine, physics or engineering. Multiple reviews and interpretative angles on research published in those areas help to protect us against ideas and inventions that may actually harm us, or that are designed for profit over health and public safety. The peer review process is our safeguard against everything from negligence to bias to simple human error.
It is also a very effective means of provoking debate and discussion among the brightest intellectuals we have in any given field. The criticism and praise given to published works makes it stronger and more beneficial for those who use and build upon such research in the future,
Depending upon how valid your research needs to be (research for your favorite animal could be much less rigid than researching literary theory for example) it could be critical to use peer reviewed material. For example the noted Dr. Ruby Payne has published a groundbreaking book called A Framework for Understanding Poverty. This book is self published and has not been peer reviewed, so there have been some academicians who question the veracity of the work. Peer reviewed material is sort of like vetting antiques. The reviewers give critiques, question flaws and weaknesses. All this is designed to ascertain the validity of the research. The peer review ensures that the source has been rigidly examined and can be considered a valid if not valuable source.
The whole point of peer review is to ensure that sloppy research does not get through to be published in journals. Articles in academic journals are supposed to be guaranteed to be worthwhile and, more or less, true. If you take articles from journals that do not do peer review, you have much less of a guarantee of the validity of the articles.
In the peer review process, people who are experts in the field that an articles is about look over the article. They make comments on the article, saying what (if any) problems there are with the work and what corrections need to be made. This ensures that the research is basically sound.
Overall, then, peer review is something of a guarantee of the quality of research. It is important to pick articles from peer reviewed journals because other articles might simply have been made up or might have been written to a much lower standard of scholarship.