The "defense" part is that big company X with a large patent portfolio might sue you for infringing on a patent - unless you have a large patent portfolio because then if they sue you, your lawyers can review everything they're doing and compare it to your patent portfolio and find ways to sue them. This is something like mutually assured destruction.

It seems preposterous to me, but this is also the basic idea I've got from working at multiple big companies where they have training meetings to explain these things. You're very much encouraged to come up with ideas and submit them for the lawyers to look at and possibly patent even if it has no applicability to anything you're doing.