I regularly find improperly installed, improperly fastened, and missing flashings while working as a home inspector in St Louis:
The purpose of most flashing is to keep water penetration from entering a home where it most likely would otherwise. It's used in places caulk won't do the job, or stand the test of time. Often, when done well, it negates the need for caulk at all. It's used to protect vertical surfaces from a driving rain, in a valley, or around roof penetrations. The top and sides of a chimney for instance, or where siding meets brick, or a roof. Another smart place to install flashing is over top of where a deck's ledger board attaches to the home, ensuring water doesn't get in between those boards and cause rot, mold, etc.
I recently inspected a home just 14 years old with NONE of these flashings installed. Siding is often a somewhat over looked trade in the overall importance of a home, but, if that installer doesn't know proper flashing techniques, as should the roofer, it will surely cause serious water damage issues in the future.
Common areas of flashing missteps I find as a home inspector:
Improperly (face) nailed roof flashing / boots
Addition attachments (roofing)
Missing drip edges
The list goes on...
In short, flashing is one of the most important, yet least understood aspects of a home, by many builders, homeowners, and tradespeople alike.