What he said^. I had an Asus Xonar Essence STX sound card and was extremely happy with it. However, when I build my new computer it was transmitting some noise from the computer and I could hear it through the speakers. It was really annoying. So I decided to get another ASUS product and went with the Essence One Muses Edition which is an external DAC. I probably would have never spent that much but I wanted another ASUS product, cause I liked the sound signature of the Essence STX and figured that the Essence One Muses has to have a similar presentation and it does.
But today's sound cards do a pretty good job of shielding and stuff like that. I guess it depends on your build/computer and how it affects your internal sound card. My Essence STX wasn't playing well with my new build so I got rid of it.
In the audio industry there is a lot of snake oil sold. A lot of those DACs that sell for thousands of dollars use DAC chips that cost literally under $10. But yet they sell the unit itself for a few thousand dollars. It's ridiculous.
But hey, it's peoples ears and money. Whatever makes them happy. Some people are happy with sound that costs less than $100 and some have to spend several thousand dollars or hundreds of thousand dollars.