# What exactly is an autoencoder? [duplicate]

I mean, if I have some MLP with an input layer of 5 neurons, a hidden layer of 3 neurons and an output layer of 5 neurons and e.g. an input like (1, 2, 3, 4, 5) the autoencoder should deliver the output (1, 2, 3, 4, 5)?! So why do I need the autoencoder at all?