The purpose of this key was to erase mistakes in a manually-input paper tape: the operator had to push a button on the tape punch to back it up, then type the rubout, which punched all holes and replaced the mistake with a character that was intended to be ignored.
ðə pˈɜːpəs ʌv ðɪs kˈiː wʌz tʊ ᵻɹˈeɪs mɪstˈeɪks ɪn ɐ mˈænjuːəliˈɪnpʊt pˈeɪpɚ tˈeɪp: ðɪ ˈɑːpɚɹˌeɪɾɚ hædtə pˈʊʃ ɐ bˈʌʔn̩ ɔnðə tˈeɪp pˈʌntʃ tə bˈæk ɪɾ ˈʌp, ðˈɛn tˈaɪp ðə ɹˈuːbaʊt, wˌɪtʃ pˈʌntʃt ˈɔːl hˈoʊlz ænd ɹᵻplˈeɪst ðə mɪstˈeɪk wɪð ɐ kˈæɹɪktɚ ðæt wʌz ɪntˈɛndᵻd təbi ɪɡnˈoːɹd.