Re vang as a safety device....
This comes from the classic 'Royce's Sailing Illustrated' (image credit)
When sailing deep downwind without a vang the mainsheet has no downward component, leaving the boom free to lift if any forces cause it to do so.
Esp. in a big sea with lots of motion, a gibe, accidental or otherwise, can turn into a goosewing (half the sail fills on one side, the other half on the other. Now the leech is a big S curve with the boom at a high angle , but eventually the sail will decide with way to go. Now the uncontrolled boom can slam violently to the extent of the mainsheet - if it's still cleated - and fetch up hard. This might break fittings, or do other damage possibly including striking a shroud.
I've only experienced this in a small sailing dinghy but thereafter we added a fixed strop as a 'vang' and it never happened again. On a larger boat the potential forces would be exponentially higher.
And I think a vang (as a safety device - fixed or adjustable) is especially important when newbies are learning.
Most of the time, agreed, it serves as a leech tension trim control, with a spring loaded or hydraulic one capable of up and down adjustment...
Sermon over......