Writing your tasks down frees your brain to think about them less

The Zeigarnik effect, discovered by Soviet psychologist Bluma Zeigarnik in the 1920s, showed that people remember interrupted tasks better than completed ones — waiters who could recall unclosed orders forgot them immediately after the bill was paid. Later research by E.J. Masicampo and Roy Baumeister found a crucial extension: you don't need to complete a task to silence the intrusive thoughts about it — you just need a specific plan for when you'll do it. Writing a task down with a time allocated is enough to 'close the loop' and free working memory.

Source: Masicampo & Baumeister, Psychological Science (2011)