It's often useful to understand the context in which a navigation happens. If we could add an initiator source_id to each source, we'd be able to gain this context, and also understand user navigation flows / user journeys (how often do users navigate between pages on a given domain? how often do they visit just one page on a domain?).

One specific high value case would be recording initiators when client-side redirects (e.g. page sets window.location) are issued. We currently treat client-side redirects as separate page loads. If we recorded an initiator, we could link the client-side redirector with the page being redirected to, which gives us a better sense of the user perceived page load performance.