It's quite simple : they do reduce the drive's recovery time, so that the drive can fit in a RAID array.
For normal drives, when there's a R/W error, the drive can enter a "deep recovery cycle" that can take up to 2 min. During that time, the drive becomes unresponsive.
Most RAID arrays do not tolerate a drive being unresponsive for more than 8-15 seconds. If it does happen, the drive is considered "faulty" and ejected from the RAID array. Plain and simple. Of course there are workarounds, but that's not the subject here.
So from that point of view, a RAID-edition drive is simply a normal drive with a recovery time that has been truncated
by firmware operation to less than 8 seconds (they dare call that "Time Limited Error Recovery", like if it was a new feature
). It's as simple as that.
Oh, and of course, there's the MTBF that goes up from 800.000h to 1.200.000h or so, with no precise explanation. Like if something secret had been miraculously added to the drives.
So well, if you believe that 400.000 more hours are worth $100, then go for them. But do not expect anything more.
Oh BTW, here's an explanation from WD. It's the same thing I've just said, put in more elegant words
Q: Regular 7200 RPM desktop drives run fine in RAID environments; why do I need these drives?
A: Unlike regular desktop drives, WD RE SATA and EIDE hard drives are engineered and manufactured to enterprise-class standards and include features such as time-limited error recovery that make them an ideal solution for RAID.
Q: What is time-limited error recovery and why do I need it?
A: Desktop drives are designed to protect and recover data, at times pausing for as much as a few minutes to make sure that data is recovered. Inside a RAID system, where the RAID controller handles error recovery, the drive needn't pause for extended periods to recover data. In fact, heroic error recovery attempts can cause a RAID system to drop a drive out of the array. WD RE2 is engineered to prevent hard drive error recovery fallout by limiting the drive's error recovery time. With error recovery factory set to seven seconds, the drive has time to attempt a recovery, allow the RAID controller to log the error, and still stay online.