Sorry, I should have been more clear about that. Pre-outs are RCA-style jacks on the back of the receiver that output from the preamp portion of the receiver. If there are no pre-outs, you're forced to use whatever amplifiers are included in the receiver forever. If there are pre-outs, you can use the receiver for switching, decoding, etc... and then use dedicated power amplifiers to drive your speakers.
The subwoofer output on the back of almost all HT receivers is a pre-out. That's fine... you hook up a powered subwoofer to the subwoofer pre-out and the amp built into the subwoofer is used to drive the subwoofer's big driver. But many HT receivers don't have pre-outs for the other channels -- fronts, center, surrounds. No pre-outs means no opportunity to upgrade amplification later -- you basically have to chuck the receiver and buy a new, more powerful (and much more expensive) receiver later if you decide you want to drive Magnepans, certain Thiels, or any other hard-to-drive speaker.
For example, my Denon AVR-2800 does everything I need it to do right now in terms of decoding (Dolby Digital, DTS, Pro-logic, etc). But its amplifier section is only rated for 85 WPC. Not bad and will drive most speakers to satisfying volumes for home theater. But wait... what if I want to use inefficient, low-impedance speakers for the left & right front channels? The built-in amplifiers might not be up to the task. So I buy an inexpensive NAD 2200 PE or Adcom GFA-555 on the used market, connect it to the left and right front pre-outs on the receiver, connect my left and right front speaker wire to the power amp instead of the receiver, and now I have hundreds of watts per channel on tap, and stability for low-impedance loads.