Lifelike Look
Many stuffed toys that detect noise levels, like the Furby or popular electronic pets, use that ability to "hear" to react properly and appear more lifelike. This creates a much more interactive experience with the toy than you could have with a silent action figure or with a remote-controlled toy. The ability to detect noise levels gives the toy the ability to operate at least partially on its own, or to appear to do so, because you activate it with sound rather than the push of a button.
Education
Many stuffed animals that can detect noise levels are also used as educational toys. The Furby, for instance, was meant to help children communicate and to teach them how to care for something that had reactions to their speech and tone. Electronic pets, which can also react to sound and sensory stimulus, may teach young children how to care for a real animal.
Commands
A stuffed animal that can detect noise is a simple mechanism, but one that can interpret words is only a step further. This automated command ability can enhance the playing potential of a toy, especially since it teaches those playing with the toys that communication must be delivered clearly and enunciated properly to get the desired result.
Cost
Since a stuffed animal that can detect noise levels is much more complicated than a simple stuffed animal, the price tags are much higher. The Pleo, a robotic dinosaur that reacts to a variety of stimuli, costs roughly $350, and according to Design Boom that's quite a bargain for this small, robotic dinosaur. As the technology becomes easier and cheaper to produce though, prices for these toys will go down.