Some soundcards can give you a broader spectrum range. This allows more lows and more highs in tue audio signal and can really improve sound. You'll hear things you didnt hear before in games and some music will sound fuller and have some instruments or sounds otherwise not heard with onboard audio.
If you are using a usb headset this wont help. The usb headsets essentially act as their own sound card. If you need optical out plenty of cards have that as well now.
Probably going to get flailed for this, but they are a waste of money. They're basically a gimmick, lose a lot of the sound quality and have high impedance. Ones onboard on the motherboard are just as good pretty much.
If you want someting to improve sound get a DAC.
If you're using optical plug it in the back. not the front.
I wouldn't say they're important. The simple onboard audio solution on your motherboard is more than good enough. And $2 vs $200 $2000 sound cards don't make a difference. You're just buying features.
get the e07k from fiio, look it up, they're cheap and good, there are probably desktop specific dac/amps aswell. Also, get good headphones first. Check out /r/headphones.
If you're using an optical out, the soundcard you're using means absolutely nothing :P. All it's doing is passing the pure digital stream and something else down the line is actually converting the stream into audio.
you know, i remember years ago certain soundcards had a noticeable effect on gaming FPS (frames per sec). i don't know if that's true anymore and i'm too lazy to run benchmarks with the sound disabled in the BIOS, but maybe if your sound is traveling through the USB that may impact fps vs a pci card