Because writing an assignment is thinking, not typing: planning sentences, checking arguments and revising wording all take time that pure typing speed ignores. Timing yourself on a real practice paragraph gives the honest number the estimator needs.

Enough that they are never an afterthought — for a typical essay, research and reading often match the writing hours, and formatting and proofing add a couple more. Entering them separately is what stops the estimate from being 'writing time only'.

Neither — the pattern that works is structure early and writing in the last third of the available time. Plan the argument and gather the sources first, then write with a deadline you set yourself, then revise. The estimator's start date is built for exactly that rhythm.