Sometimes you can't tell a shingle is loose and no longer protecting your home unless you check.
Water Damage on Ceiling
When water leaks through the roof, you can often see it start to leak through your ceiling.
Roof Decking Damage
This is the damage to the roof decking in one of the leaking spots. This wood is covered by the shingles, so you wouldn't know there was a problem until you notice a leak and investigate.
The part where both sides of the roof meet in the middle is called the ridge. This one was splitting and allowing water to leak into the home, but you wouldn't know unless you were on the roof.