The main reason is QUALITY. We perform MP3 compression with professional algorithms that are not available in consumer software. Professional MP3 compression provides superior quality. Another reason would be CONVENIENCE.
"Thanks for your work on my recent order. As an audio engineer, I understand how hard it is sometimes to 'retrieve' good sound from old vinyl from the '50's and '60's. (hell, we all used to roll joints on those double albums ;-}) Just wanted to say thanks for your good work!"