It depends on the person, some people do not find a point for them. I personally used to be one of those until I got mine, and I can tell you I can hear a big difference in sound quality (and I only have a $50 headset) and also in sound positioning (CMSS-3D actually works) while in games. (On this tactical night-mod server on COD4 I was able to track down my enemies pretty accurately by closing my eyes and listening for footsteps)
I suggest that if gaming is your primary concern (like it was for me) that you get an X-Fi card, possibly the cheapest decent creative card, the X-Fi XtremeGamer.
Creative 70SB046A00000 7.1 Channels 24-bit 192KHz PCI Interface Sound Blaster X-Fi XtremeGamer Fatal1ty Professional Series -
Dont do it for inbuilt tv speakers, i dont care how good you think they are... they wont be.
Main reasons for getting an addon soundcard is for added connectivity (becoming less and less of an issue these days with onboard having digital in/out) Support for codecs and encoding/decoding... and better sound quality. However, if the speakers arent any good the quality will not go up.
It really depends on the quality of the output device, headphones or speakers and amp. If you're gaming a Creative card can theoretically speed up gaming by ~15% (but this is widely debated). For your TV's speakers, even if they seem pretty good, they're probably not (unless truly marketed as being superior...and even then I'd be skeptical). I don't think you'll hear any improvement from buying a sound card. I'd not be surprised if your TV's amps/speakers have such a high THD+N and low S/N ratio on them that adding a sound card for better sound is just an exercise in spending money.
Now, if the TV is marketed as having superior sound quality (SQ) and you want to get the most out of that you'll have to test it out for yourself. I'd not recommend buying anything more than, say, a Creative Extreme Audio for a few bucks and I think that's way overkill for your application, except that it might ease your mind.
For instance, the Dell 32" LCD we use in the bedroom was marketed as having really great sound. Well, it sounds better than what is on most TV's, but your standards would have to be pretty low to be really impressed by it from a SQ perspective, despite Dell's marketing attempts. The speakers are still not high enough resolution to let you hear great SQ so adding a soundcard to improve the SQ is useless. However, a soundcard might get you some cool DSP effects and a contemporary, non-Extreme Audio card from Creative could potentially speed up gaming.
Again, the truth is that onboard sound quality is really good enough unless you've got mid-to-high end speakers+amp/receiver so don't waste the money.