Greetings,
Is there a way to SET a Particle.Variable? I see how to GET a variable’s value and display it through console, html, etc… Or is the only way to do this by running a Particle.Function that changes the variable value within your code?
I would believe that is the case. 
Keeping in line with the REST standard, a POST would be used to, well, post data. A variable can’t be changed directly remotely.
Copy that, thanks Gentlemen!
As you said, if you want to set a variable on the device, you’d normally use a Particle.function() and there you could do what you want with the variable you exposed as Particle.variable()
But it’s not the only way. You could also do that via Particle.subscribe() or loads of other ways.