More seriously, ADHD, among others, is a working memory disorder. A way to cover this is to use "prosthetics" for it, such as a notepad that you always carry with you. If this affects you strongly, train your muscle memory to use it to note down what you were planning to do and to refer to it when you forgot.
In PC-speak, ADHDers have traded RAM for more CPU.
Writing something down increases the likelihood I will remember. It feels like there is something about listening to information that makes it difficult to remember and retain. I often forget what I was saying or lose track of what other people say in conversation.
The process of writing may activate some other part of our minds, or that because writing is slower it becomes easier to retain in memory. Any event with a heightened emotional state becomes far easier to remember. Quite mysterious.