why must i download sims from origin if i have the disc

I purchased the sims3 game disc. I have origin installed on my computer and an origin account. i am struggling to install it because when i click install after instering the disc, it 'adds' the sims to 'my games' in origin then when i click on the sims3 game, it says i must download the game. that game file is 6.1GB why must i download the game? what is on the disc if when i click install, it says i must download? please explain to me how sims3 must be installed from disk step by step.