Quieter and smoother are always good, but they are especially good when noise is a critical factor - such as in audio and video development. Western Digital is announcing plans to breath more life into developing drives custom-fit for just this purpose, aimed at situations like DVR/PVR systems, CCTV systems and other such that require writing large files, often simultaneously. The last thing you want when recording a show (assuming you're there to watch it) is a 10k RPM drive screeching in your living room, and WD's drives will offer technologies to combat just that:

They include a a new technology called IntelliSeek which synchs head movement to disk spin, so the head arrives at the right moment to read the data it wants as it passes underneath. This does make them slightly slower on average seek time - 12 or 13ms versus 8 or 9ms for a PC drive - but Patterson said that by taking out "hasty movement", it can cut both power consumption and noise.
They also will be incorporating technology used to reduce noise in mobile drives in these ones as well, such as automatic head parking. Along with fans and optical drives, hard drives often stand out as the noisiest component in a system. Unfortunately, that's the nature of magnetic storage. Flash promises to change a lot of that, but is still very expensive.