This is classic war fare and not terrorism. The Saudi's have made a mess of the conflict in Yemen, which is more or less part of a bigger proxy war that also involves the US and Iran. On paper it's rebels vs what passes for a government there locally. But in practice it's Iran sponsored weaponry vs. Us & EU sponsored weaponry. If you look at a map, the reasons are obvious. The Saudi's are connected to international waters via two narrow straights that border on Iran and Yemen. Yemen is strategically relevant to Iran for this reason because it allows them to isolate the Saudis.
What Iran is demonstrating here is that they can put the squeeze on oil supplies. The wars in the middle east are fundamentally oil centric. It's about who gets to supply oil to the world in exchange for vast amounts of dollars. The ideology gets the headlines usually but if you follow the money, you end up in Riyad, Teheran, Moscow, Washington, and Tel Aviv. Iran is making a very basic point here: we can shut down your oil any time we want.
It's a clever move since it puts everybody on the spot economically. In related news, Trump is planning to meet Rouhani. I'm guessing they have a lot to discuss.