lots of people say they're not needed but if you want better sound, toss one in. you'll need to know what kind of expansion slot you can put one in ( and have room for ) pci-e 1x/pci/pci-e 4x-8x, etc.
Kinda like putting your speakers on a more powerful amp compared to on board sound.
There's little reason with the modern onboard sound to get a sound card - it does very little unless you're a fairly picky audiophile.
The only reasons I would install one:
1. I got it for free (which actually is the case for me, I got a good one free)
2. I had a specific need for a certain type of input/output that only the sound card could provide.
You're unlikely to hear any difference unless you have a crazy speaker setup (THX certified and beyond) and are a careful listener/have good hearing.
That said, they are very easy to install if you've ever opened a PC (provided you have an open slot..and it's not a small form factor PC).