do you need to download steam games?

Answer 1

technically no,

if your logged into an account and connected to the internet you can restore a backup of a game install created through steam

or if you have a friend with the files you can copy them into the correct place and steam should pick up the files. you may need to start the download, exit steam, copy the files over, then restart steam but this can be fussy to get working

if you cant do either of these then yes you need to download