It really comes down to how picky you are about how pure and clean, crisp of punchy you like your sound or music.
And if you have a good Amp and speaker setup to complement a add in Pci-e based graphics card, some of them costing you for the audio card alone in excess of $200 or more.
For most of us the sound chips that are provided on new motherboards are sufficient and up to the task of producing good audio output Gavsta.
If you do any kind of Daw work, creating your own track or recorded work , songs.
Then it is another case as to why you may opt for a single Pci-e based sound card.
For many people there is no need to bother with any add in audio card, though back in the old days of motherboards the sound produced by add in cards was far more superior in the quality of audio output the y could produce, because Company`s who made motherboards opted for cheaper lower quality sound chips to be soldered to the motherboards to keep cost of the overall product down in price.