1) We need to keep files sizes manageable so that it does not take a long time for the recipient (A&R / Music Supervisor / Producer / Director) to download your music. If it takes too long, they will not download it and move on.
2) 160 kbps is a fair compromise between good quality and small file size.
3) Many people have file size limitations on their email accounts.
4) Storage capacity.
MP3 files are a universal format that can be played natively on all computer systems, PC, Mac, Linux, etc…by default, you do not need to download any 3rd party application to play them.
AIF files allow you to embed ID3 metadata tags into the file. The allows you to copy the file to another computer or format (MP3, AAC, MPA, etc) and transfer the information without having to retype it, this cannot be done with WAV files.