A reviewer is a expertise who can modify the things and give advise on work and future work including the manuscript/book evaluation. He is almost like adviser of a skill.
One preferably review the article of his/her own area of research. By doing so, one will come to know that what the other researchers are doing in the same field. One can suggest that how the things can be improved by his/her experienced views. By going through the article one can also suggest the future studied. So to my mind, by REVIEWING an article you not only made suggestions to the author but you may also be benefited by knowing many new things about which you are not aware earlier. I think these are the benefits of being a reviewer.
Reviewer is a person who will look at the work of others and adjudge the suitability of the work for publication, in an area of his/her expertise. In doing so, he/she not only advises the authors how to make the study more rich in terms of the scientific content but also benefits himself/herself also by knowing what is new in the field of research. During the process, he/she is offering voluntary service and in return getting knowledge about the present scenario of research. It is not possible many times to get time for reviewing a manuscript out of the busy schedule. But as a part of scientific community, a researcher can contribute in the least possible way by acting as a reviewer. As far as the benefits are concerned, it is only self satisfaction I think. Some journals will publish the names of the reviewer in their print issues. No monetary gains are provided as far as I know. Acting as a reviewer, he/she is required to provide comments on the suitability of publication to the editor as well as the authors. If revised, he/she will be required to look at the manuscript again for the problems which he/she pointed in the manuscript and to see whether the manuscript meets the criteria for publication or not. Sometimes reviewers are invited to submit their own papers also.
Being a reviewer myself, it has taught me editing and writing skills that translate to my own manuscripts. Additionally, reviewing allows you to hone your critical thinking skills and teaches you how to efficiently relate your thoughts to others. I highly recommend it if you get the chance.
Reviewing any work is itself a brain tonic for a researcher, I think. While reviewing you experience that new ideas just occur to you. Being a Reviewer is like taking up a responsible task because your duty is to competently review the work in such a way that nobody should be done injustice including science. You are responsible to Editor, Authors & Science community as well.
Advantages of being a reviewer are mostly intangible in nature such as one comes in contact with new ideas and methods in his/her field, improves himself in reasoning and scientific writing, it's a credit to your resume (though it may be unethical to disclose your participation as peer of any specific journal).
Science journals never offer you with monetary benefits for reviewing the paper. On the other hand, you will have the following advantages:
(1) If you receive an invitation to be a reviewer, that means, you are considered as an expert in your field. It is an honor for you.
(2) Editorial board of any journals consists of senior professors and well known scientists in the respective fields. As a good reviewer you will have a chance to get recognized by those editorial board members. This will also pay you when you apply for grants or for academic jobs. Since many of those members will be in selection panel.
(3) You can include reviewer experience in your CV/resume.
(4) By reviewing several papers in your field you will be growing professionally by updating your knowledge and expertise within the field.
(5) Additionally, you will become aware of how journal and publication's works in your field and how the editor makes a decision.
(6) Finally, you are making some significant contributions to the scientific world.
(7) If you are applying for a green card (permanent resident) in USA, being a reviewer is one of the criteria that you can meet.
I may suggest using PUBLONS (https://publons.com/home/) and it is one of the best professional ways to keep track of your contributions as a reviewer and obviously it is an easiest and most trusted way to show your records of your reviewing activity. You can share the link of your Publons profile in your CV instead of writing the reviewing detail. Also you can monitor some of the key points mentioned below by using the stats option in your dashboard page in the PUBLONS.
· The distribution of the Journal Impact factors of journals you have reviewed.
· A cumulative record of the total number of your reviews.