Many of our students do not have an accurate, internalized sense of time. The teacher tells them that they have 10 minutes to work on something, and when they get the 1 minute warning, they are shocked! They had no idea that ten minutes passes so quickly. A visual timer, projected on the board, provides them with an easy system for keeping track of time.
As a teacher, I also benefit from the use of a visual timer. As students are working, I am roaming the class and supporting. I frequently get caught up in something a group is discussing, or helping a struggling student. If I don't use a timer, before I know it the 10 minutes I had planned for has turned into 15 minutes, and I am now trying to catch up with the rest of my lesson plan. A timer keeps teachers on track as well as students.
My favorite visual timer is Timer by Ten. After it counts down to 0, it immediately begins tracking the elapsed time until it is stopped. The elapsed time shows up in red digits (see photo.) I can then use this data with my students to talk about how long their transitions take. I have even bar graphed the data and motivated students to transition more quickly each successive day.
If you are connected, there is also a great web tool that is free - Online Stopwatch. It has a variety of visuals and sounds to add some novelty to your time tracking.