The most common form of this is probably 'journaling', that is, writing a little every day about what is on your mind. This is a way of working things out for ourselves, reflecting on issues, planning, noting insights, making lists, anything at all. It can take the form of prose, mind-maps, lists, dialogue, clustering, or writing a letter. It can happen in the evening, to record the day's events, feelings and lessons. In the morning it can be used for reflection, meditation, or planning.
Many writers use a form of journaling as a workshop and the storehouse of their raw material.
In the bookstore there are books by Julia Cameron, Natalie Goldberg and Gabriele Lusser Rico who focus on writing for yourself - and then writing for publication. This is an entirely different kettle of fish. In fiction, the mix is different. The author must have something to say, often called 'the premise '. Without this, most readers will drop the ball and wander off.