As an influencer you can give reviews about brands with whom you have worked with, but brands can also give review about you. After finishing a campaign reviewing with stars is compulsory, but commenting in text is optional. After a brand has given a review about you you cannot delete or alter it, it stays the same. This is information for other brands on what is it like to work with you - whether you are reliable, punctual etc. When you review a brand you can take their response time, their kindness and a lat of factors into account which helps other influencers when working with that specific brand.
You can get References after working with a brand. The advertiser can set this as an option in a campaign, this way a campaign can be publicly listed for you as a reference. Other brands can see these references on your profile - this is good because they can see how great it is to work with you. So you should always make sure that a brand is satisfied with your job and gives you good reference.
Reviews also affect your content quality and response rate score!
Brands can see your received reviews on the website.
And the references from you previous works.