We will still be buying stuff from the EU. We are not suddenly going to stop

Of course not, but we will be free to buy stuff from the rest of the world too.
We can choose whether we apply tariffs or not, and we can stop buying EU stuff that is more expensive than alternatives.

And WTO tariff rules are not a free-for-all, by the way.

They are freer than the restrictive EU, and we already do most of our trade that way anyway. It works very well.