In the beginning of recorded sound, there was mono. One speaker, period.
Mono speakers were plopped wherever it was convenient, and that was that. Consumer audio remained strictly mono until the late 1950s with the introduction of stereo tape and LPs. Now you needed two speakers.
Home theater upped the ante to 5.1 channel surround sound--five speakers, plus a subwoofer--and setup hassles were getting tricky. Dolby's Web site offers very specific requirements for the placement of the front left, center, right speakers, and the side surround speakers. 6.1 and 7.1 systems add rear surround speakers.
It's one thing to look at a diagram, but your room probably doesn't look like the diagram. Reality sets in, so very few 5.1, 6.1, or 7.1 system buyers get remotely close to the recommended speaker placements.
I've seen countless 5.1 home theater in a box systems in real people's homes with all five satellite speakers clumped in a row under or over the TV. Some buyers spread the speakers out across their entertainment furniture, still with all the speakers in front, near the TV. Obviously, those people don't want to string wires across the room. I don't blame them.
On one hand it'll sound "fine," but the envelopment the film sound mixers worked so hard to achieve will be lost. Don't worry, the Dolby Police won't arrest you for improper placement and the certain destruction of the filmmaker's intent.
If you have all of your speakers sitting in a pile, but I've made you a little curious, temporarily move the surround speakers out into the room. Put 'em on something to get them off the floor: A chair, bookcase, furniture, and so on. Play a few big action flicks and see what's up with surround. It might surprise you and just maybe you'll be inspired enough to make the effort to find permanent, around the room locations for the surround speakers. Hey, in 5.1 it's only two skinny wires.
What about 7.1 channel home theater? Most new AV receivers are 7.1, would a properly setup 7.1 system sound better than 5.1? Maybe.
Seven channel systems work best in large rooms, say 17 feet by 24 feet or larger. The whole idea of 7.1 is to supply a more complete wrap-around sound experience than 5.1. However, when the speakers are closer together in a smaller room, the extra speakers are less important. Especially when the sofa is up against the rear wall--there's no way to achieve behind-the-listener sound anyway. If you want your surround system to sound as good as it can, run the receiver's auto setup/calibration program.
In any case the differences between 5.1 and 7.1 are usually pretty subtle, it's not like adding those two extra speakers makes a huge difference in perceived sound quality. It's not worth agonizing over.
Do you have setup questions for us? If so, leave them in the comments.