I don't know that the Signalink has all that much better soundcard specs than other devices... I am guessing its similar to the $9 usb sound sticks you can buy these days which are pretty good themselves for the money.
The biggest benefits of the signalink are:
1) Is just the fact you have an additional sound device.
So you are not in danger of being one of those guys who xmit windows system bings/bongs on 14.070
By providing a seperate dedicated sound device, It frees up the built-in sound card for said bing/bong system alerts, email dings, etc...or other purposes such as listening to online streams, or using echolink or whatever you want.
2) It makes cabling much much easier. A single USB cable instead of all the others normally needed for psk connections. I guess thing might help with signal noise by not having audio cables running all over the place and picking up some power supply, compuer hash or other ambienet noise.
3) The fact you have knobs. Sure, you can use built in level controls via software sliders etc..but it is nicer to be able to just reach over and adjust the levels with your hand while you watch the ALC meter on your rig, and not have to watch a slider on a screen at the same time.
Really, the signalink is a nice device. Maybe a tad overpriced for what is a $9 usb soundstick and a little bit of extra circuts for doing PTT/VOX(even though I disabled the VOX in mine and use CAT), but its well made, and they handle interfacing for your to various radios well.
I would say its a fine piece of ham gear.