What is the benefit of using force download

Thanks JayAdra,

That's what i needed to know exactly.
Actually I'm trying to sell video files.

Yes, you're right maybe the user will copy the link and then paste it anywhere , but how its possible to only Play the video on website?
Should i use the same idea as you mentioned ?

i saw some websites like educational websites, they do this(we can only play on the website and no download or link!)