This is really something that would be hard to do without. It makes it a cleaner task to fill your jars. Keeps the rims cleaner.
You must still wipe the rims to be sure you have not spilled, but there is less mess using one of these.
Not just for canning. I like to store leftovers in canning jars. The clear jars make it easy to see what I have. The funnel is useful for filling jars.

I can't imagine doing this neatly can you? What a mess it would be. Of course it is kind of messy anyway... but imagine without the funnel.