Seigi - I had a grommet pull out in a bad storm years ago when I wasn't using a ridgeline. This led to the tarp flapping around a lot and opening us up to the weather. By having the ridgeline there is very little tension on the grommet to the tie off point. There's also a better chance the tarp won't cave in should a branch fall on the ridge during bad winds. While it may bow in, the odds are better that it won't collapse on you.
That's all for now. Take care and until next time...Be well.