Some furnaces use the ignitor as a flame sensor.
After the ignitor cools down after getting white hot from alternating current,
the furnace controller looks for a tiny direct current flowing from the “grounded” hot burner metal frame to the ignitor.

Troubleshooting this design of “flame sensor” can be VERY challenging as you need an electrical meter able to read micro amps of direct current.
Soot buildup and corrosion can both mess up proper operation.

Some “universal” furnace control computers like the Carbide 50E47-843 from Emerson allow you to use either type of flame sensor, or convert one to another.