So a proof might claim to prove a theorem about a whole class of numbers or other mathematical objects. But if there is bug in one of its lemmas it would mean its results are true of only a subset of the mathematical objects claimed. That would be like getting an error on specific inputs.

The proof then remains a valid proof of something although not exactly a proof of everything its authors claim it to be a proof of.

An erroneous proof is also usually useful because it is not clearly wrong in most cases since nobody has come up with counter-examples so far that would prove the proof incorrect, yet.