Realtek onboard (even the latest) is inferior to a half-decent addon sound card.
I compared my new Asuz Z77 Sabertooth's onboard audio with my older Auzentech X-Fi Forte and there's a HUGE difference in gaming, playing movies, and music.
*However you also need quality speakers to justify the onboard sound card. I have M-Audio AV40 speakers.
So it's not a matter of "need" as pretty much all motherboards have onboard sound (which you disable in the BIOS if you buy an addon sound card), but a matter of "wanting" the best quality.
I might recommend a $30 Asus Xonar DG51 sound card and $90 M-Audo AV30 stereo speakers.
**Another issue is headphones:
If you use the onboard audio it's pretty simple to stick your headphones into the front case headphone jack and disable the rear output to the speakers automatically. If your sound card has only one output this is an issue. Sure you could plug the headphones into the speakers (if an option) but you'd likely require them to be powered on.
A lot of people claim that onboard is "good enough" and anything more is "overkill"; that's really only true with $40 and below speakers, better speakers will sound better with a sound card.
I should add that you can get a SPLITTER for 3.5mm jacks to go to both headphones and speakers. So that:
a) you can TURN OFF your speakers when not needed (make sure the ON/OFF switch is on a speaker even if you get a subwoofer)
b) Turn Volume down or Unplug headphones when not needed. Some headphones have volume control, some may even turn off. Some really need to be unplugged so you may want a cable running to a convenient spot (or use the speaker output but that adds power).