I don't really have many examples of "how" it can be used. I only know the one use case for why it was requested. In that case, the developer wanted a tighter integration but something to where they could pass some data and then immediately update some records in the database on delivery of the screenshot. So, they are passing a "listingID" and the instant the screenshot is delivered, they are noting that so the system can perform an action and notify the user. This way, they don't have to rely on "polling" their screenshot storage location for the screenshot and will know the instant it is ready. This eliminates the need to "poll" our service (with the notify callback) and the need to poll the user's storage (by correlating the custom data in the notify). Generally speaking, this is overkill for most sites that can just check for the screenshot when someone visits a page or requests it, but it makes for a very fast integration for screenshots for time-sensitive implementations.

That is the basic usefulness of passing custom data, but there may be other creative uses we haven't thought of yet.