The main function of a sound card is to accept audio into the computer and output it again, preferably with the minimum of delay (latency) in between. It is never advisable to connect analog signals to a computer, either input or output, as there is too much electrical noise around. But for digital input and output, a basic sound card can give good results.
However, even though a sound card might do this to a perfectly acceptable standard and genuinely earn the tag 'professional', it is still generally up to the computer's processor (CPU) to provide the horsepower for multitrack recording, EQ and plug-ins. Processors are optimized for general computing tasks – word processing, still images, Internet access etc. Although the processor can work with video and sound, it doesn't do it particularly well.
The case of video is a parallel worth investigating. Take a computer of reasonable quality with a standard graphics card (or maybe graphics processing actually on the motherboard). Play your favorite action game and observe the jerkiness of the motion, blocking and other artifacts, and generally a low grade visual experience. Now swap the graphics card for a state-of-the-art, top quality board. Suddenly the action is smooth and almost photographic in quality.
Similar principals apply to sound. Basic recording and playback of audio does not require additional processing other than that provided by the main CPU. However, plug-ins soon eat up that power. For serious recording, even the fastest processors are not really up to the job. Reverb is a particularly resource-hungry process.
The solution to this is to buy a sound card that has horsepower of its own, such as the TC Powercore Element. Now, the burden of processing is taken off the CPU and handed over to chips that are purpose-designed to handle audio signals. The result is that you can run more plug-ins, plug-ins can be of higher quality, and in all probability you can record more tracks, depending on hard disk speed.
But recent developments are hinting that these specialized and expensive audio cards might not be necessary. A graphics card has all the power you could possibly need – measuring typically at 40 gigaflops compared to around 6 gigaflops for an Intel or AMD processor. (One gigaflop is a billion floating point operations per second).
There is no reason, given the appropriate software, why this power could not be devoted to audio rather than graphics. So apart from input and output, you wouldn't need a sound card.
So look at your computer again. It is struggling with audio. But right there inside the case is an audio engine that is as powerful as the best. Except right now, you just can't use it.
This is certainly an area of development worth looking into.