Although this is not strictly necessary and is not part of the IUPAC naming system, the usage is still common in cases where one wishes to emphasize or distinguish between the straight-chain and branched-chain isomers, e.g., "n-butane" rather than simply "butane" to differentiate it from isobutane.
ɔːlðˈoʊ ðɪs ɪz nˌɑːt stɹˈɪktli nˈɛsᵻsɚɹi ænd ɪz nˌɑːt pˈɑːɹt ʌvðɪ ˈɪjuːpˌæk nˈeɪmɪŋ sˈɪstəm, ðə jˈuːsɪdʒ ɪz stˈɪl kˈɑːmən ɪn kˈeɪsᵻz wˌɛɹ wˈʌn wˈɪʃᵻz tʊ ˈɛmfɐsˌaɪz ɔːɹ dɪstˈɪŋɡwɪʃ bᵻtwˌiːn ðə stɹˈeɪttʃˈeɪn ænd bɹˈæntʃttʃˈeɪn ˈaɪsəmɚz, ˈiː.dʒˈiː., "ˈɛnbjˈuːteɪn" ɹˈæðɚ ðɐn sˈɪmpli "bjˈuːteɪn" tə dˌɪfɚɹˈɛnʃɪˌeɪt ɪt fɹʌm ˈaɪsəbjˌuːteɪn.