Useful biochemical energy takes the form of ATP synthesis and maintenance of an electrochemical potential via a proton gradient--higher proton concentration outside a cell membrane than within.
Electron transport chains occur near cell membranes, as proteins embedded in the membrane are crucial to the process. In eukaryotic cells, ETC takes place in mitochondria.
The ETC is biochemically linked to other processes vital to cell function that wouldn't occur by themselves. Such thermodynamic coupling allows cell and equilibrium and function to maintain.
Electrons high in potential energy in an electron donor go through reactions with various complexes to lower potential energy, ending in a terminal electron acceptor (oxygen).
The Darwinian stance argues chance events worked out such that the ETC offered a competitive advantage to organisms endowed with it. Another line of reasoning is that instead of resulting out of probability and competition, the ETC was an emergent property of natural laws, therefore inevitable.