Traditional vertical rows can help to control behavior such as cheating and talking and minimize classroom disruptions. If you have a particularly unruly class, consider moving your students into vertical rows with adequate space between desks. This format allows you to place chronic disruptors into strategic spots in your classroom; just be sure to leave enough room between desks so that you can move freely among the students.
Vertical rows also make it easy to learn and memorize student names. Many teachers typically start out the school year with students in vertical rows, arranged in alphabetical order. This traditional format allows the teacher to easily connect students to his seating chart, and, as a result, allows the teacher to identify and remediate behavior issues before they escalate.
Most teachers incorporate some type of cooperative learning, or group work, into their lessons. Teachers who primarily use the "vertical row" format in their classroom may find it inconvenient or impractical to assign students to groups. Conversely, teachers who have their students sit with groups, whether at tables or in clusters of desks, can easily transition in an out of cooperative learning activities. You can choose to rotate students through groups, so that by the end of year, students work with each person in the class (and, possibly, even form new friendships).
Unless you are the type of teacher who feels comfortable moving up and down vertical rows as you teach, the traditional classroom setup can allow shy or disaffected students to "hide" in the back. Conversely, if students sit in a circle, or in groups of three or four, they may be more likely to participate simply because they feel more accountable.