Sometimes you just want to lie down and watch movies from the comfort of your bed instead of sitting down in front of your computer. If you have a laptop, go nuts, bring over the laptop to your bed and watch away. If you have a desktop, though, good luck with that. If you have a mobile device, just load it up with your movies and you're set, right?
But sometimes you simply just run out of space on your device. Sometimes people opt to be more economical and choose say, an 8 GB model and pocket the savings. Or it could simply be that you have more content you want to put in your device than what it could hold. Between apps and music alone, your device could already be reaching its max capacity, and you still want to shoehorn in a whole season of Game of Thrones to watch before you go to sleep.
Why not stream those bad boys and save space instead of reshuffling your device's files over and over? If you have a Wi-Fi router in place, then you're almost there.