Why is it important to eliminate the garbage qubits?

[bytebuster]

Why is it important to eliminate the garbage qubits?

Most reversible quantum algorithms use standard gates like Toffoli gate (CCNOT) or Fredkin gate (CSWAP). Since some operations require a constant |0⟩ as input and the number of inputs and outputs is equal, garbage qubits (or junk qubits) appear in the course of the computation.

So, a principal circuit like |x⟩ ↦ |f(x)⟩ actually becomes |x⟩ |0⟩ ↦ |f(x)⟩ |g⟩,
where |g⟩ stands for the garbage qubit(s).

Circuits that preserve the original value ends up with |x⟩ |0⟩ |0⟩ ↦ |x⟩ |f(x)⟩ |g⟩

I understand that garbage qubits are inevitable if we want the circuit to stay reversible, but many sources claim that it is important to eliminate them. Why is it so?

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