Why is electrochemistry useful?
See the discussion that follows.
Electrochemistry, which includes oxidation-reaction reactions, has a large number of practical applications that are of importance not only in industry, but also to "everyday" experiences.
Examples include the operation of all batteries, both rechargeable and not. It includes bleaching, antisepsis and electrolysis (an enormously important means of obtaining metals from ores).
This is just a short list of applications I could think of at the moment. Hopefully it gives you some idea of the range of reactions included here.