If your projector is to be ceiling-mounted, the power cable in the pack will be rather too short. Longer power cable will be need to power up the projector, up there where it is mounted. That's why you need this power cable.

Just like you may also need a VGA cable. Most projectors come with a regular-sized (short) VGA cable in the carton. If you need to install that projector, the included VGA cable can hardly get up to where the projector is mounted on the ceiling. You need a longer VGA cable. you can choose the right length of VGA cable with this link.

If this 40m power cable is longer than what you require, you can check the 20m power cable here.