In the case of MP3 the compression is considered perceptual. This means that for example I was listening to a song on the radio, I would not hear all the sounds that were played. As a function of my ear I would only hear the most prominent tones and not the entire range. You ear is just not accurate enough to hear these sounds. This is called lossey compression and the name is fitting.
T hat is a good question.
I mean ISDN, DSL, Cable Modem etc. The internet and its usefulness is partly determined by file size. Hence whatever it is becomes useless because it can't be used. MP3 offers a manageable file size that the common net head can handle, and download. Over a cable modem you could download a song in just under a minute You will be able to down load entire albums in 15 min.