Usually when some one says onboard sounds the same as a sound card, they usually used generic headphones or speakers with said sound card or never really heard good audio..Besides having a good source, Having good gear to go along with it also matters. I also agree with nameless,As there still a need for sound cards as onboard audio devices still isn't up to the level as good sound cards, no matter how much improvement realtek does to the chip.
It's a shame really, they could be doing much better with putting audio on the motherboard,But when they do, The motherboard cost like 2x more then the mobos without it. Plus It be something different if he was gonna connect his optical out of his onboard into a external dac amp. But like nameless says, the onboard audio might not have the features that the person would use for gaming like Dolby headphone or cmss3d, Which all would be sent over optical to the external dac/amp. Like how I did with Xonar DG's Dolby headphones when I sent that Thu optical to my NFB-12 for games. In my case if you like me and your onboard audio drivers are glitched, where you can't even use it optical as it doesn't work correctly,Then a cheap sound card with optical saves you the headache.
I sent a friend of mind that same Xonar DG and he been using onboard audio for years, As soon he gave it a listen as he didn't expect it to do any thing. He fall on the floor in shock, It was his best Christmas gift ever.