Due to the nature of SeattleWiki, a lot of the pages may be fairly location-specific. For example, there may be pages about certain topics for a particular neighborhood, or pages about a particular shop, restaurant, park or theater that happens to be in this or that neighborhood.
Subpages are useful when you want to create a page about a general topic, as it pertains to a particular place. Example topics could be homelessness, transportation, or places to eat. Since each neighborhood might have a page on that topic, we need to somehow differentiate the pages for each place. Subpages let you do this by basically creating "child" pages of main articles. Subpages have the name of the main article followed by a "/" followed by the name of the page. Capitol Hill/Eating cheap, Lake City/Homelessness and Lake City/Transportation and Parking are all good examples of subpages.
See also the Wikipedia article on subpages.
Adding location to page titles Edit
Pages that are about particular places, like shops, restaurants or parks, don't need to be subpages, since it's not likely that there will be two of the same name. Nevertheless it's useful to put the location in the title, since that makes it easier to find them in category pages. For example, take a look at Category:Movie theaters or Category:Bookstores for examples of why it's useful to include the location name in the page titles.