Why, well the software environment you are using dictates:
ability to interface with different bits of kit (i.e. supported control surfaces & I/O),
how the software processes information (DSP card vs CPU cycles i.e. protools HD vs well, the rest of the world) which translates directly to performance and ability to handle plugins / track count before dropouts,
ability to use different plugins (VST vis TDM etc),
ability to handle midi,
Audio formats supported (AFI vs Wav),
ability to handle MP3s / other compression formats,
number of supported midi instruments,
number of supported tracks (many of the freebies will only support 8 tracks etc),
midi sound modules supplied / supported......
and these are just the ones that spring off the top of my head.
To say that a free bit of software is the same as say Logic pro 7 (clocking in at a cool thousand dollars) is just not true.
Converters and clock as well as I/O in general are all important as is mic choice and placement / preamp choice as is ability to monitor and the acoustics of your recording environment. That said a recording will only be as good as the weakest link in the recording chain, and that includes the performance you are capturing and your own abilities as an engineer !
page and www.studiotrax.net
for all things recording. my toons
Brought to you by Dr BadGAS