Essay as Exploration
Essayer — French "to try." Montaigne's 1580 essays established the form: an essay is an attempt, not a statement. You don't know what you're figuring out yet; you can't begin with a thesis because you may never arrive at one.
This is opposite to the school essay model (inherited from law school rhetoric and English lit), which requires a pre-chosen thesis defended through supporting paragraphs. A real essay starts with a question — noticing a door ajar and walking in.
Why write instead of just think?
Expressing ideas helps form them. Most of what ends up in essays is only thought of when sitting to write. Writing for readers sharpens this: inviting people over forces cleaning the apartment; writing for an audience forces thinking well. Ideas written purely for oneself peter out — they run into difficulties, grow vague, and drift.
The defend-a-position failure mode
Defending a position misses subtleties and prevents changing the question. School essay structure — topic sentence = thesis, supporting paragraphs = blows, conclusion = restate for jury — is derived from legal training, not inquiry. Good writing is convincing because it's right, not because of clever arguing.
When a draft seems unconvincing: talk it over, find a clearer explanation — or change the claim. The aim is truth, not winning.
What it produces
A real essay is a cleaned-up train of thought. Real thought is full of false starts; the essay cuts and fills to emphasize the central thread, like an illustrator inking a pencil drawing. But don't change so much that you lose spontaneity.
Related: meander-principle (the navigation method within exploration), learning-by-writing (similar thesis — writing forms thought, not just records it)