A sound card is not essential for gaming. Onboard sound is adequate for most users. If you are an audiophile, and/or have a really high end set of speakers or headphones, you can get better quality sound with a discrete sound card. Sound cards are hardware upgrades.
Whether it is worth spending the extra money for a high quality sound card is up to you. The cheaper ones will be similar to what you already have on the motherboard, and are meant more as replacements for broken onboard sound, rather than an upgrade. Higher end models will give you better sound quality, but only if you have high quality speakers. If you have $20 desktop speakers or headphones, you won't hear a difference between sound cards.