The Identity gate is a single-qubit operation that leaves the basis states and unchanged.
For simulations, this gate is useful in combination with the
error_model instruction. Because it represents a specific action on a qubit (do nothing), the depolarizing error model will take this gate into account when distributing errors on the qubits. This can be used as a simple version of decoherence of the qubit.
Note that more sophisticated error models might be needed to model more accurately the effects of decoherence, leakage, etc.