Kinda glad you asked that question because it made me do some research. If the current avi videos you have work, I see no real reason to convert them.
Based on a few google results, it seems that both AVI and MP4 are "container" formats. AVI has been around for years and has limitations. MP4 is newer and "seems" to support more audio and video formats. Comparison of container formats - Wikipedia, the free encyclopedia
I am by no means an expert in this. If anyone else has more information, please speak up.