You do not need your PS3 connected to the internet to install the game data.
what installing the game data does is install roughly about 5gig's of the data on the game disc onto your PS3's hard drive ,speeding up loading times as the hard drive can be read faster than the game disc . the only reason not to do this is if you are short on space on hard drive.
Just go to the options menu scroll down to the bottom section " miscellaneous" select "utilities" then select " pre-install main game data " click "ok" and prepare for a LONG wait as the installation can take up to an hour to complete. you only have to do this once and it will speed up the games loading times alot
