I never claimed that it's cheap, but this is usually what people with several TB of storage at home are doing.
$2.50 is cheaper than having to buy another disk if something happens to it.
It's for the convenience of it. If I have a movie library of hundreds of movies. I can find one in an instant and play it on the big screen on my HTPC. I can be at a friends house or a relatives house and pull out my phone and stream it from my server at home and connect it to their TV with HDMI. I can give out a login to friends and family and they can enjoy streaming the movies too.
Then of course there is the piracy option too. $2.50 to store a BluRay is far cheaper than buying one haha.