Look at a map again. Iran is a powerful state in the middle of the same Eurasia where Putin is building his union. An alliance of some sort with Iran isn't strictly required, but it's certainly helpful. At the very least, Putin wants good relations with the Iranians. And he wants America and American-friendly regimes away from his underbelly for the same reason he wants them off his western flank in Europe, where he fears the West and its economic and military alliances might encroach.

Moscow on the Tigris: Russia Joins the Terror Nexus (19 February 2016)