Iran's military has been very intelligently shaped to maximise its ability with limited resources against the threats it's most liable to face.
If Iran had focused on developing the best tank force or air force possible, would it make a difference? No, Iran's most likely enemies are either the US or a country supplied by the US and the best Iran could produce would still fall short. They would certainly lose a conventional war.
So Iran has focused on developing asymmetric warfare capabilities. Their very large infantry and special forces mean any invading force will have an extreme insurgency problem to deal with, which could keep any enemy bogged down in a bloody and expensive war for literally decades. And their ballistic missile force can target important economic, military command or political targets, adding another significant deterrence.
If Iran had focused on developing the best tank force or air force possible, would it make a difference? No, Iran's most likely enemies are either the US or a country supplied by the US and the best Iran could produce would still fall short. They would certainly lose a conventional war.
So Iran has focused on developing asymmetric warfare capabilities. Their very large infantry and special forces mean any invading force will have an extreme insurgency problem to deal with, which could keep any enemy bogged down in a bloody and expensive war for literally decades. And their ballistic missile force can target important economic, military command or political targets, adding another significant deterrence.