Overview
Modern hard drives feature an ability to recover from some read/write errors by internally remapping sectors and other forms of self test and recovery. The process for this can sometimes take several seconds or (under heavy usage) minutes, during which time the drive is unresponsive. RAID controllers are designed to recognise a drive which does not respond within a few seconds, and mark it as unreliable, indicating that it should be withdrawn from use and the array rebuilt from parity data. This is a long process, degrades performance, and if more drives fail under the resulting additional workload, it may be catastrophic.
If the drive itself is inherently reliable but has some bad sectors, then TLER and similar features prevent a disk from being unnecessarily marked as 'failed' by limiting the time spent on correcting detected errors before advising the array controller of a failed operation. The array controller can then handle the data recovery for the limited amount involved, rather than marking the entire drive as faulty.
Read more about this topic: Error Recovery Control