Mainly because our brains have evolved to be really good a pattern matching but not so good at organising and arranging.I left my TV at HT because I was tired. But I dreamed about a Poch post-match interview where he explained why we list.
Making a list allows us a visual representation of a number of items that we can then reorder and prioritise as required. They also serve as a handy method of recording the status of said group of tasks.