But for now, let's take a boat. I know nothing about boats but I can imagine a wooden boat with a cracked hull.

I would imagine you could look at that crack and in many cases deduce what stresses it had been under, for how long and what else might be affected. That's similar.

No one stabs another person 20 times simply to kill them. That indicates a lot of anger, either toward the victim personally or what the victim can represent. Narrows the field as most people aren't capable of that.

In violent person-to-person crimes it's often easy to tell if the perp is right- or left-handed, or tall or short.

Profiling is far, far less accurate than a polygraph, for instance, but still very useful.