system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Whenever I buy a video game disc, (Xbox, Ps4, etc.) It says it needs to download the game into the console. Isn't the game already on the disc?
gpt
On modern consoles it has to install the data from the disc onto the console's hard drive. That's because blu-ray drives are just too slow to load data for modern games, so it has to be installed on a faster (although still pretty slow to be honest) hard drive.

Usually you also have to download patches, even if the game has just come out. The data that goes onto the disc has to be ready months before the game comes out. So in the time between that data being ready and the game actually coming out the developers continue to work on fixing bugs, tweaking the gameplay, maybe even adding features. So when the game actually comes out there is an update ready and waiting to be downloaded.

In theory if the game is single player you can choose not to download the update and just play it anyway. But unfortunately some games are pretty broken without that day 1 patch.