Benefits of volunteering for peer review
If we work in a particular subject area and like a particular journal related to our subject, then many benefits can be expected by serving as a volunteer reviewer.
These are summarised by Wiley (an academic publisher) as being positive for your discipline, your own research, your career and your understanding.
Of course, the benefits will not always be immediate. One benefit may be learning better how to write, by seeing the problems in texts written by others.
Ideally an author will seek review before submitting a paper, so that the reviewer can give advice or at least an opinion on a suitable choice of journal. For this, authors need to look for supportive colleagues and teachers in their own field.
My impression as a reviewer is that inexperienced authors often have no clear vision of who their readers should be, and do not seriously consider whether a journal will reach an audience that is relevant for the planned paper.