So when we were kids, most of us got amalgam or "silver" fillings at some point. Silver fillings can last a really long time but when they decide to go, they go down in a blaze of glory.
So when we started to remove the filling and see where the cracks went, the entire corner of the tooth literally fell off without any effort. The cracks had run across the floor of the tooth and to the other side. Luckily, the tooth fractured above the bone level and didn't involve the nerve so it can be repaired with a crown. Worst case scenario means the tooth needs to be removed or needs a root canal to save it.
If you catch them on time, you can fix them with a filling or sometimes a crown. If you wait too long then you run the risk of needing a root canal or losing the tooth. And we don't want that!
Get them checked!