Some fly-fishers will take this to the extreme.  Before tying on they'll investigate by disturbing the river bed and scopping up what's there.  Depending on what they find, they'll pick a fly that most closely matches what's in the water.

Finding a hatch in progress is often the jackpot.  Even if you can't match the hatch, a shallow retrieve around the hatch just might match the baitfish feeding on the hatch and net the lunker feeding on the baitfish.