As with the choice of hardware, the software used to play can be considered a fit and forget, one-time choice, but if you decide to experiment with different software, the files will not be altered by the investigation process. Endless arguments rage over which software is best, but the differences are usually ones of taste and functionality rather than performance.
In every other way, a computer audio source behaves like just about any other source. The analogue output (ideally from a DAC) connects to analogue inputs on your amplifier or preamplifier. The change this computer-side system is having on the industry means sometimes even the DAC is built into the amplifier itself, and we expect this to become more commonplace.
The fact that you can do all this with a computer is wonderful, but what relevance does it have to good audio? One of the biggest misconceptions currently in hi-fi today is that computer audio means compromised audio. It's not hard to see why; many people consider computer audio to be synonymous with MP3 and those same people generally consider MP3 to be synonymous with low-quality MP3. While there are a lot of low quality MP3 tracks in circulation, this does not represent the entire story of computer audio; companies like Linn and HDtracks are now delivering music at master tape quality levels. But even with files that are as good as the original master tape available, the question remains; why should I bother?
There are many answers, but two of the most important are convenience and quality. Computer audio systems give you near instantaneous access to your whole music library. This has a fascinating effect on the way you listen to music. Although you still end up listening to complete works at a time, you also tend to investigate your music collection more thoroughly, as everything is to hand. On systems like the Sooloos for example, you can 'swim' through your record collection, picking out tracks connected to one another by drummer, producer, orchestra...you name it.
Then there's the quality issue. Many feel that CD files sound better when freed from being played direct from the CD, because you are not relying on on-the-fly error correction and because you aren't relying on an opto-electronic mechanism prone to vibration issues. But CD is only the start; high quality downloads offer potentially SACD and beyond sound quality in the home. This is something that only a few years ago was the stuff of dreams; music companies selling the Crown Jewels for not much of a premium. Now, it's becoming a reality. Again, expect to see such things blossom over the decade.
So maybe, the answer to the big computer audio question is not 'why?', but 'why not?' Computer audio will be a key theme in hi-fi of the second decade in the 21st century. It begins...now.