The authors rely on reviewers to be constructive, reasoned, explicit, and ethical. You should be sure to identify both the strengths and the weaknesses of the paper. You should be aware of your own biases or preferences. Don't be superficial or dismissive. Other scholars rely on reviewers to maximize the quality of research published in their field.
Journals are perhaps the single most important vehicle for dissemination of research findings.
Remember that you are setting the standard for quality. Being a manuscript reviewer is one of the best and most effective ways to continue your own education as a scholar.