Trade covers your shortfalls, but it depends on peace holding and your routes staying open. Self-sufficiency is the other side of the coin: the less you depend on the outside, the less a blockade or a war that suddenly cuts your routes can hurt you.
Why It Matters
A nation that depends on trade for its steel or oil is vulnerable: all it takes is an enemy cutting that convoy route, or you going to war with your supplier, for your industry to start failing at the worst possible moment. The closer you get to producing at home what you need, the less room you give the enemy to strangle you without firing a single shot.
Synthetic Refineries
You can build refineries that produce synthetic oil from your reserves, or synthetic rubber without relying on plantations. They're not free — they consume industrial capacity and, in the case of synthetic oil, carry their own costs — but for a nation with no natural access to those resources, they're the difference between depending entirely on the outside or having a cushion of your own.
Resource Extraction
Resource extraction technology increases the output of deposits you already have in your territory, without needing to conquer another inch of map. It's one of the most cost-effective investments in the medium term if your problem isn't "I have none of this resource" but "I have a little and it's not enough."
When It's Not Worth It
Investing industrial capacity in synthetic refineries or extraction has an opportunity cost: those factories could be building military equipment instead. If you have stable access to trade and aren't at war or about to be, sometimes it's simply better to buy the resource you're missing and put your industry toward something else. Self-sufficiency becomes more worthwhile the closer you are to a long war or to being isolated.