Technically SAM systems are purely for defence but to get a complete set to defend you need a lot more than just anti-air.

Defence deterrent is based a lot on potential counter attack so if you don’t have a counter attack potential it’s kinda safe to attack you. Nuclear bombs are an example of this, they are the most aggressive and destructive which makes them very effective for deterrence.

One thing Ukraine has shown is that fortifications are super important which are purely defensive. But to guard a long defensive line without a massive standing army you need a crapton of drones which can be use both offensive and defensive.