There are two ways you can get goods: you can produce it domestically, or you can import it. If a country doesn't do either, it doesn't get the goods (meaning that no one who lives there will get them, either).
Now, trading only occurs if the following are the case:
- You have something to sell that other people want*
- Other people have something to sell that you want*
In most cases, money is used to facilitate this process, but you could also use other goods. Here's the problem, though: if there's nothing available for me to buy that I don't already produce myself, why would I care about getting more money? There's nothing I can spend it on, so it's basically just unicorn points. In this case, I'd have no reason to trade - I'd just use any goods I produce myself.
Conversely, if I have nothing to sell that other people are willing and able to buy, I'll have no way of getting money, so I won't be able to trade even if I want to - I'll be limited to consuming whatever I produce myself. Furthermore, if I can't sell it anyway, I might as well use it all; granted, I may still want to save some of it until later rather than consuming it immediately - e.g. to have enough food to last the winter, or in order to have seeds for next year - but the point is I'll eventually use all of it.
At this point, you might ask: what if I have a really good year and can't finish all of it myself before the food spoils? Well, in that case, throw a party for all your friends. (Or, for that matter, feed someone who has less than you). Why not? There's no reason not to consume all of the output before it goes bad.
This analysis, of course, ignores services.
* Technically, we should say "are willing and able to sell" and "are willing and able to buy," but we're simplifying a bit here.