This was to guarantee there would be some lunar soil brought back in case an emergency required the astronauts to abandon the EVA and return to the LM.
ðɪs wʌz tə ɡˌæɹəntˈiː ðɛɹ wʊd biː sˌʌm lˈuːnɚ sˈɔɪl bɹˈɔːt bˈæk ɪn kˈeɪs ɐn ɪmˈɜːdʒənsi ɹᵻkwˈaɪɚd ðɪ ˈæstɹənˌɔːts tʊ ɐbˈændən ðɪ ˈiːvə ænd ɹᵻtˈɜːn tə ðɪ ˌɛlˈɛm.