Just like the drive chain, timing chains elongate, fatigue, and wear out. Luckily, they are not subject to dirt and mud, are bathed in an oil bath, and their overall environment is much better. Before I get into it, one misconception I want to clear up right away is that the timing chain doesn’t technically stretch. Instead, the pins and rotating elements of the chain wear. When the pins wear they become smaller and their mating holes grow larger leading to increased clearances and chain length.
When an engine is run with a worn timing chain engine performance is compromised and the likelihood of related failures is greatly increased (think chain tensioner). Cam timings that are off several degrees will result in a loss of power and the cam chain tensioner will have quite a job trying to take slack out of the valvetrain. When a timing chain elongates it may not do so in a uniform way and parts of the chain may be tighter or looser than others. While automatic cam chain tensioners have proven to be reliable on the majority of engines, some model years, brands, and individuals have fared better than others. A worn timing chain which adds extra slop and inconsistent chain tension to the valvetrain certainly won’t make the tensioner’s job any easier. So it makes a lot of sense to keep tabs on the condition of the chain itself from time to time.