Here's why Audacity .....
Audacity is used because it is Open Source (GPL) and therefore cross-platform. It works on Windows, Mac, Linux, Solaris and BSD -- all from one tar.gz file. This simplifies the logistics (you don't need to put umpteen different pre-compiled binaries on the CD, just the Source Code tarball) -and- the technical support (it looks and works the same way on every OS). For automated track-splitting on non-Windows platforms, you should investigate Gramofile ( http://www.opensourcepartners.nl/~costar/gramofile/ ) -- and don't be put off by the thought that it hasn't changed much in years (nice to run something with a version number past 1.0 occasionally!). But it's actually not that hard to do track-splitting manually in Audacity, as long as you've a fast processor and plenty of RAM.
Many PCs don't seem to have an analogue line input anymore, just the (low impedance, high-sensitivity and -- the real show-stopper -- mono) mic input and a headphone/speaker output. At any rate, even if you do have a line-in port, the sound quality from a sound card built into the motherboard will suffer because the A-to-D converter is sharing a power supply with the noisy computer, and is highly sensitive to noise carried on the power supply lines; and because noise can also be picked up by inductive and capacitive coupling from the various high-frequency signals on the motherboard. A PCI sound card might well have some extra filtering to guard against this, but will still never be perfect. An external A-D converter with its own power supply, remote from the digital circuitry, should theoretically have the best noise immunity. (Of course, if they build it with lousy components, it will still sound lousy.)
Strictly speaking, you don't even need an RIAA preamp for vinyl records. If you've a mixer with high-sensitivity, high-impedance inputs and tone controls on each channel, you just need to boost the bass and cut the treble. This is actually how I've done my most recent analogue rips.