I use a rating system when reviewing anime, I put it at the end of my reviews after explaining in words why I like or dislike a particular show and certain aspects of it that appeal to me. Rating systems have been used in all of media to determine an objects value, usually rating it out of 10. The object whether music, film or anime is rated to determine how good the show is in comparison to others of its kind.
First Reactions are a big thing in the anime community, as soon as the first 3 or so episodes of a show are out, people are quick to share their thoughts on the show or predict the shows tone and future. It is very common for a show that is currently airing to shoot straight up the top 50 or even top 20 position on most anime list such as MAL or HummingBird before it slowly starts to fall back down after it is done airing. This is because of 2 things: 1. Most people reserve rating a show until it’s done, and the only people who are rating it whilst it is airing are mostly fan of the source material, and 2. More people like to binge watch a show after it is done airing to avoid cliffhanger.