I've had a Yamaha soundbar for a couple of years and the instructions gave advice on positioning.
It's close to the TV and underneath/slightly in front of the TV. You really need to position it so it seems that the sound is coming from the TV....
OG...Many people find that the sound quality from the speakers in flat-screen TVs is pretty poor (because of their small size and odd position)....hence, the popularity of soundbars. The difference in sound quality can be substantial...like comparing listening to radio broadcasts on a conventional radio and listening on a good hi-fi tuner/amplifier and decent speakers. As to whether it's worth the cost...depends on your ears and how good a sound quality you'd like.
If you want to listen to your TV through hi-fi speakers, you normally have to connect the TV output through your hi-fi system, and not just the speakers. Then, the sound is probably coming from the wrong direction ie where your speakers are, unless you move all your gear around!