[sveiss]

The disc acts as a licence key. The game itself doesn't get read from the disc after installation--that would be far too slow.

The disc versions of the consoles are popular for people who like to buy games second hand and/or trade in after they've finished playing; it's frequently much, much cheaper than digital purchases, even when the digital versions are on sale. There are disc rental services like GameFly, too.

Of course, the manufacturers would prefer to kill this secondary market, so sooner or later I expect the disc drives to go away completely. That was Xbox's plan around a year ago, per some recent leaks, and if one does it the other certainly will as well.

[nubinetwork]

I could understand if it was just a key check, the drive could spin down after starting the game... except it doesn't. The laser stepper is pretty quiet compared to the motor, so I can't tell if it's actively seeking data.

[extraduder_ire]

Concern about people pulling the disc out after starting the game, maybe. Pulling out discs without the console realizing has a long history on playstations.