The WI-FI box and the modem box are two different things.
And cable companies NOW have their own versions.
You don't have a choice.

The cable that comes into your house first hooks to the modem box.
Then it hooks to the WI-FI box that sends the signal through the air and throughout your house.

If you don't have a WI-FI box the modem hooks straight to a desktop computer with a cable.
But laptop computers need that WI-FI box because they do not have a direct hook up.
And can you carry them around like a cellphone.