Many Iranians are complaining about Iran helping Hezbollah in Lebanon, often emphasizing that there are enough poor and needy people in Iran more worthy of support.

While it´s true that there are indeed many poor people in Iran, we should keep in mind that Irans financial problems are not due to money „wasted“ on funding Hezbollah but mainly to – largely unjustified –  western sanctions. Irans loss from being disconnected from the international payment system and from the extreme sanction based decline of foreign direct investments is in the tens of Billions.

Hezbollah is Irans extended front line with Israel. Without this „artificial border“ Iran would not be able to deter Israel from attacking Iranian facilities by making use of US provided long range bombers.

Thus, when the civil war in Syria broke out and took a clearly sectarian tone by attracting foreign Shia- and Iran-hating Jihadists, both Iran and Hezbollah understood the existential threat. It was no coincidence that Israel immediately supported the „rebellion“ in Syria (while at the same time treating stonethrowing Palestinian youth as „terrorists“).
Irans support for the Syrian government is neither because of the former being led by an Alawite (often wrongly called a „Shia sect“) nor with the purpose of expanding Shia Islam or suppressing Sunnis. If Irans motivations were „sectarian“ then why did the country support Sunni Afghans (Massouds Northern Alliance) and Arabs (Hamas)? Why the support for Sunni Europeans (Bosnians) in the Balcan wars?
Irans support for Syria has three main reasons:
1. During the Iran-Iraq war Syria supported Iran, while all Arab middle east and Gulf states supported Iraq with money and arms, sometimes even with fighters.
2. Syria shares a border with Israel and constitutes another remote front line for Iran in case of a war with Israel.
3. Syria is the only land route to Southern Lebanon. Without an Iran friendly government in Damascus Hezbollah would not last long in any conflict.

The departure of the Syrian army from Lebanon in 2005 marked the rise of Salafi militants in that country. These forces have at times not only attacked Hezbollah but also engaged the Lebanese army.
As early as in the first months of the start of the Syrian war Salafi militants from Lebanon were intruding Syria and attacking the police and armed forces.

Iranian military strategists recognized the threat immediately: A sectarian insurgency enjoying the support of western powers, Israel, Turkey and the Gulf States, getting arms, funds, equipment and training from Saudi Arabia, Qatar, Turkey and the CIA while being romanticized and whitewashed by western and arab mainstream media would overpower the Syrian government. It was only a question of time.

As predictable as the pending fall of the Syrian ally was, it was also clear that the various backers of the insurgency shared one motivation: hatred of Iran and – as far as the Gulf states were concerned  – the Shia.
Iran could not afford to wait and see waves of foreign Jihadis arrive in Syria to not only „liberate“ the country from the „Nusayri infidels“ (derogatory term for Alawites) but in a further step move on to defeat the „Rafidhi“ (derogatory term for Shias) Hezbollah nearby in Lebanon.

What would happen next?
Since 2003 Iraq has been experiencing years of relentless bombings and massacres against the Shia majority (mostly civilians and including Sunnis living among Shias) carried out by radical islamists, many of them Arabs from Gulf countries. To make things worse Al Qaeda in Iraq (AQI), presumed dead, resurfaced as ISIS and intensified devastating terror attacks and warfare both in Syria and Iraq.
It was not far fetched to assume that after defeating the Syrian army and Hezbollah Syrias sectarian insurgency  would export the emerging „caliphate“ to Iraq to fight and defeat the Shia government. Despite the Shia making up some 70-75% of the Arab Iraqis the fall of the formally Sunni Saddam government was a thorn in Saudi Arabias eyes and continues to be hardly acceptable even 14 years later.

Iran had and has no interest in having hordes of sectarian „Majoos“ (derogatory term for Iranians used by Arabs) hating islamists on its borders. The decision to dispatch Hezbollah to the Syrian battlefields was nothing but the correct anticipation of an upcoming deadly menace to Irans security and territorial integrity.
In Syria Hezbollah continues to suffer casualties but has managed to contribute heavily to the survival of the government and the rolling back of the jihadists. Hezbollah engaged and defeated both Al Qaeda and Isis in Lebanon as well as on Syrian battle fronts. Without Hezbollah fighting Isis near the Iraqi border in eastern Syria the Iraqi army would have a much harder time defeating Isis in Mossul.

Hezbollahs proven capabilities in assymetrical warfare are a major reason why so far Israel has refrained from attacking Iran.
Plus, as mentioned, Hezbollah managed to severely weaken the anti-Iranian, predominantly Salafi insurgency in Syria and choke off any domino effects leading to the reestablishment of an anti-Iranian government in Iraq.