it warms the cylinder bore and the pre chamber so the air is hot enough for compression ignition


diesels operate on compression ignition

wheras gas cars are spark ignite (spark, fuel, air)

diesels (compression, fuel air)

the glow plug just warms the air before the initial start and stays on for maybe a few seconds after.. after that its a controled explosion limited by the fuel unlike a gasser where its limited by the air intake.