Back up dimmer for T-stat?

[fluffpuffgerbil]

So tomorrow my new Hydrofarm thermostat should be showing up. I see people using back up dimmers with their T-stats incase their t-stat fails.

And I have a dimmer I'm using now so to let it not go to waist and ensure the safety of my snake, how exactly would I hook it up? Or is it just a, *it failed let's hook up the dimmer* type of thing?
Or can you explain to me what a backup dimmer actually does? XD
Thanks!