On Xbox at least, downloaded games seems to cost more than the dvd versions. Largely because the store continue to charge full price for many games, when after a while dvd games are dirt cheap.

Downloaded games require the appropriate services to be still there to be able to play. Eg the device that checks your licence. Having said that, more and more dvd games now require Xbox live connection too now :(

If you have a disc you can always put it in the machine. If you want to redownload you are at the whims of publishers leaving it available in the Xbox store.

https://www.dafoot.co.uk/index.php/computer-juice/gaming/180-xbox-game-prices