While low compression readings may be a sign of poor cylinder condition, it may also be nothing to be alarmed by. A borescope allows the A&P technician the opportunity to easily inspect the cylinder and valve heads without removing the jug*, saving both time and money.
“A compression test could tell you that air was leaking past the exhaust valve, but with the borescope you could tell whether it was because of a benign glob of lead on the seat that would quickly resolve itself the next time the engine ran—or a malignant, warped or eroded valve likely to fail catastrophically in the next 10 hours…” – From AOPA Pilot’s Savvy Maintenance by Mike Busch (Borescope ascendancy: Time to topple the venerable compression test? May 1, 2017)
* What’s a Jug? Answer: a very technical term that refers to an engine cylinder on your airplane. But you already knew that 😉