Useful writing tells people something true and important that they didn't already know, and tells them as unequivocally as possible. Useful writing makes claims that are as strong as they can be made without becoming false.
The product of four components determine usefulness:
Novelty, is it interesting and new to the reader?
Correctness, while keeping precision in mind
Importance, a product of the numer of people something matters to, and how much it matters to them.
Strength, by thinking well and the skillful use of qualification
Tips:
To write an essay, write loose and then tighten.
Don't publish an essay if you have not learnt anything from writing it.
To get novelty, write about what you're thought a lot and anything that surprised you.
Don't underestimate qualification. It's an important skill in its own right, not just a sort of tax you have to pay in order to avoid saying things that are false.