same as you don't take on the ocean without a map of the seas, you don't start your day as a programmer without a clear plan. the plan is basically a text file with the list of tasks you would be working on during the day and the timeboxes for those tasks.
so while working on those tasks, you would have a clear idea of the progress you made that day and you would be able to adjust your pace accordingly. while investigating or programming, one roams around the codebase a lot and often gets lost. having such a list quickly reminds you of the priorities. read a timer is better than a clock for timeboxing to learn about timeboxing.