There are strategies that involve attacking a strict neutral. The Allies (US/UK) can take Spain to make the US supply line towards Europe shorter. The Allies can also build an IC there and can fight its way liberating France to reach Rome and Berlin. You can only do this if the UK controls the Middle East and Africa. Otherwise Germany or Italy can take Turkey and take control over the Middle East (lots of NO’s for Germany/Italy).
The Axis can also attack strict neutrals (for extra money or an easy corridor towards the Middle East. If they do so, the US can take Chili, Argentinia, Venezuela (2 IPC’s each) so if the Axis decide to do so, make sure you attack as of the struct neutals you can, at least Sweden, Turkey and Spain. This way you prevent the Allies trom taking a bunch of free infantry and IPC’s. Also: make sure you attack it with force so you’ll have enough to take and hold it.
Most games i would not recomend you to attack a strict neutral, but you can use it as a suprise strike when your opponent doesn’t expect you to do so