What is the 'job' of the byproduct water in the Electron Transport Chain?
Thanks for your question.
One reason is, if this chemical reaction does not happen so there will be a lot of electrons in mitochondria in a short time consequently the environment inside the mitochondria will be charged in negative and as we know the environment inside the all parts of cells should not change during the time
and on the other hand, these extra electrons may carry out some chemical reactions which result in producing some materials which are not good for cell, for ex H2O2, as we know it is a poisonous material for cell.
But maybe you ask why H2O and not another product? because H2O exists in cell, in a large amount, it is solvent for many materials, it is not poisonous for cell, and many other reasons.