It doesn't have to be. I don't think there are sufficient empirical studies to suggest writing by hand is better in some dimensions than typing.

I prefer it myself for several reasons. First, it forces me to filter the present jumble of ideas in my head into a linear progression on the page. Second, it helps me resist the temptation to edit myself before I have finished. I tend to be more mindful and focused when writing long-form.

Writing digitally, which I often do, comes with the temptation to edit and succumb to distraction before I've finished a thought. It allows me to explore many branches quickly... but quantity is often not what I'm after.

I think what you consider sufficient has more to do with your personal process and what you hope to achieve with your writing, if that makes sense.