Streaming implies that a server is able to send enough data to the client so that the client can generally play the media without waiting for the majority of the file to transfer. With video - especially movies - this technology is fundamental for watching longer form movies. Transferring a multi gigabyte file in seconds is quite expensive, whereas having a dependable stream of data means the download only has to be seconds or minutes ahead of the position of the movie being seen and for a show of one hour - the download can take one hour and one second and be fast enough for uninterrupted playback.
Streaming also reduces the storage needs on the client. It only needs to have room to cache the content needed for the upcoming stream and not have space to contain the entire movie.