According to Parkinson's Law, the longer you have to complete a task, the longer it will take you to complete it. For example, if you have 30 minutes to write an email, you will probably finish on time. However, if you have 3 hours to write it, it'll take you three hours. That's why timeboxing can help with productivity, as you will have a limited amount of time to finish a given task before the next one has to start.
To learn more about Parkinson’s Law, check out this short video about it!