To me, strictly personally ? I do store a number of guidelines for creating my own music, the music I like to hear. For instance, for several years now, I have found the notion of cellular transition (my own term) to be astonishingly productive in the matter of dynamic improvisational form. In other words, using this internal guideline tends to produce sounds of the kind I like to hear. If it ceases to do so I shall not use it.
An important point is that my personal theory (call it that for the sake of argument) is constructive and dynamic, not descriptive and static. That is to say, its principal effect lies in producing music, not in describing music which already exists. I think this distinction is very important. We may construct all manner of descriptive theory about the appearance of a flower, but such detail, however complete and accomplished, does not lead us to understand how a flower evolves from its biological instruction.
To take a more direct example from the forum, Derek recently posted videos about mental methods he employs to improvise the sound known as "baroque". These are essentially personal constructive theories, dynamic guidelines. Their results sound good to him, and probably to others, therefore he nurtures them. They are dynamically useful to him. They have little in common with the huge body of static descriptive theory which no doubt exists for baroque music.
It isn't that one is "better" than other, I'm not iconoclastic about centuries of scholarship, but it does depend on what you want to do. As a boy, I bought Tovey's analysis of Beethoven and actually read most of it. It is an admirable work of tremendous scholarship, but it is a strictly descriptive and static theory. If I actually wanted to imitate Beethoven (I don't !) I would immerse myself in his sounds and form constructive, flexible, dynamic guidelines of the sort Derek is using for his baroque.
Those are two meanings of "theory" for me.