Why do video clips have to be completely downloaded before they start playing?

The reason for this is most likely that an .AVI format has been used. The AVI file format does not allow streaming from web, which means the entire file must be downloaded to the client computer before playback can start.Choosing a different format such as WMV or MPG will allow streaming but can affect both size and quality of the video signal. WMV is usually the most size-efficient format, whereas MPG is the most standards compliant.