The majority of manuscripts that journal editors receive are unsolicited. Some journals, however, only accept papers that they have invited. Some manuscripts will be of extremely high quality, but others papers will be borderline in terms of the scope of the journal and quality of work. With any paper submitted you will have to decide whether this is what the readers want or need and this is where peer reviewers come in.
The quality of peer reviewers is extremely important to the quality of a journal. Peer review helps to uphold the academic credibility of a journal—peer reviewers are almost like intellectual gatekeepers to the journal as they provide an objective assessment of a paper and determine if it is useful enough to be published.