If you need me to explain why a "better" transport "is" a better transport then I guess you need to speak with an English major lol.
It's like asking why a tt that spins the record more accurately, more immune to outside influences, etc matters. The dac can be thought of as a cable extension, no matter how good it is, crap in one end crap out of the other.
Ok let's look at this a different way, take two identical cd players, set one on top of a paint shaker while playing, which do you think will sound better? So, it seems that the heavier, more solidly constructed, better built, better laser assembly, etc etc transport will, well, transport better. It's actually pretty simple.