There's just something about writers and cities. In a book in which so little happens, the setting in which things do happen is important. Aside from her visit to Oxbridge and Fernham, Mary Beton never leaves London. London is also where all the excitement is: Judith Shakespeare escapes to London because it's the only place she can gain the experience to write her plays and poetry, and Mary Beton has a vision of the London street that inspires her with the idea that she needs to finish her essay. In A Room of One's Own, London is a big "machine" or a "factory" and everyone, even a woman writing at a desk in her own room, is an employee (2.2).
Woolf forgot to mention that a writer's room needs to have a window that opens out onto a lot of action.
Without the stimulation of the London streets, Mary would never have been able to finish her argument.