What is the purpose of the electron transport chain in the thylakoid membrane?

To produce energy in form of ATP

Explanation:

The thylakoid membrane harbors photosystems that will receive light photons, ejecting electrons from a main chlorophyl molecule in a reaction center, to other acceptors.  These electrons will reach the electron transport chain to create a proton gradient, and subsequently, to produce ATP.  Later on these electrons will reach the other photosystem, to produce reducing power.  This is in plants.

Cyclic photosystems also exist, in some bacteria, for example,  and only produce energy as ATP.  They also have an electron transport chain.