Proper use of +page.js, +layout.js

[AlanBreck]

Beyond a load function, and defining page options (prerender, trailingSlash, etc.), are there other appropriate uses of those files? I was thinking they could be used to export some constants, but it seems to be discouraged by the need for a _ prefix, which makes me wonder if the +page.svelte (or layout) is the appropriate place for that kind of thing.