What are glow plugs used for?

Heat. When you put the battery on it it glows. After you take the battery off when the engine starts, there is a catalytic reaction with the methanol in the fuel and it continues to glow and contribute heat.

Jim

I have a chain... when you dangle it, it acs as a glow plug....

Does that mean you have to have a glow plug to start the engine?

Yes. It's equivalent to the spark plug in a gasoline engine. As Jim said, you hook a starting battery to the glow plug, spin the engine, and it starts(if fuel and air mix are correct). Once started, remove the battery and the engine will continue to run; the glow plug element is made of a platinum alloy and will continue to glow from combustion heat and catalytic reaction with the methanol in the fuel. The plug screws in to the engine head just like the spark plug in a gas engine.

Here's a glow plug and a glow plug igniter....(small battery inside)

....here's what happens when you connect the two.

That red hot glow stays like that, even when you disconnect the igniter, as
long as the engine is running.....it keeps "glowing" like that, and the engine
keeps on running.

FBD.

Along with other factors like fuel mix and compression, the glow plug can be used to adjust timing in your engine. Generally, the hotter the fuel used, the cooler the glow plug should be. Conversely, with a low nitro fuel you generally use a hot plug.
One symptom of a mismatch is, with the needle set properly, when you remove the booster battery, the engine slows down slightly. To compensate you can: Up the nitro, up the compression (adjust head shims), or use a hotter plug.
Try the plug first because this is also a symptom when your plug is becoming fouled with a coating of something. If you can't flush it off, you have to discard the plug.
In winter some flyers up the nitro a bit because they are already using a hot plug.
Get to know your plugs well and you may be flying while others are fiddling with their starter (or flipping).

George