This question is asked by many gamers on the internet and no one seems to have a proper answer for this. Well the need of sound card for gaming depends upon some major factors and also on users personal preference. If your integrated sound chip is not of good quality and is not compatible with latest games then you need to spend some cash to get a decent sound card for your system, but if your motherboard comes with a decent onboard sound chip then there is no need for you to get a sound card. Nowadays almost all motherboard comes with good onboard sound with features like 5.1 channel surround sound, 24-bit support, digital output and various effects and settings in their drivers software. Some good onboard sound chips are Realtek, VIA and Creative (manufactures soundcard and speakers also).
Some people argue that a dedicated sound card can increase performance in the games by taking off load from the CPU as the sound card itself has its own processor unit for processing various sound functions. This is true when you use some of the advanced sound settings in game options, but if you only use the basic sound settings then you will not notice any performance increase in games, whether you use sound card or onboard sound. Also a sound card with a dedicated processor chip is very expensive and costs as much as low-mid range graphics card. Instead of getting an expensive I would recommend getting good quality speakers that will enhance your gaming experience.
So the bottom line is that it not necessary to purchase sound card for your gaming needs because of the tremendous improvement of onboard sound solution over the years. But if you have extra have cash flowing through your pockets then you can have a go at it.