Yeah I don't think the 494 has pre-outs, but the 595 does, I think. The pre-outs (which I just learned from here the other day) are simply used to connect your receiver with an amp.
The pre-outs on a receiver allow it to be used as a processor in conjunction with separate amplification. You'll find many people that start with a receiver and decide to move to separates (dedicated processor/controller and amplifiers), and some prefer to add amplification but don't want to purchase a separate processor.
Just to elaborate a bit on what everyone Scott said:
In a receiver, the flow of informtion is (1) processor, (2) pre-amp, and (3)amplifier, in that order. (Note that in the case of music only, you could bypass the processor if you used the DACs in your CD player. In this case, an analogue signal, rather than digital, would enter the receiver and go directly to the pre-amp.) The "pre-outs" on the receiver allow you to send the information from the pre-amp to an EXTERNAL amp, thereby bypassing the amplifier inside the receiver. In this case, you are using the receiver as a preamp-processor.