I guess you won't "need" an amp for those headphones. The card is supposed to have some juice, I got mine out of curiosity basically. Now that I have, I really love it. The sound is very rich, clear and even more refined than before.
There are many threads here on headphone amps. You might want to read one of those to get the picture of what they do and why someone could want to use one.
The most basic application would be that you can't reach a desired volume or that your playback device does have really bad sound distortion at higher volume levels which you'd know are not because of bad speakers/phones.
I chose mine because of the nice review I read here, the very compact size of it and it's pleasing simplicity