What is the benefit of review a scientific paper as a referee?
As mentioned in the other answer, reviewing is an important service to the community.
However, you will also profit yourself from reviewing: You will learn about a new topic. You will learn about how a review process works. You get to see the other side, which will most likely helpful for your own future paper writing: Since you know how reviewing a paper feels from the perspective of the reviewer, you can better understand how to make a paper accessible to reviewers and readers, and how to best respond to reviewer comments.
And, in addition to these benefits, you can indeed list in your CV that you review for journal X, Y, and Z - this does not only show that you are willing to help the community (which will recflect positively on an application), but it also shows that the editors of the journal consider you competent to review their papers.