Video/Audio/Sound/MP3 download/watching is very resource consuming. You Bandwidth, memory and time.
but some times when you watch a video on the internet and after that you think you want to download the that video but issue is you again need bandwidth and time to download that video.
But you can download the video you just watched/streamed on your computer without requesting the server again. so it will save your time and bandwidth.
e.g you watched a video IPL 2 cricket match. its size is 30 mb. after watching it you want to store it in your mobile or computer.
you just have to use flashGot plugin in your mozilla firefox browser. it store your stream data. you need to just click on the icon. it will download the data to a file. it its already streamed but its half the way, it will request only rest of the video rather then full video.