Sūrat Yā Sīn provides a clear contrast between two reports by the messengers. The first contains “إِنَّ”; the second combines it with the emphatic lām.
إِنَّا إِلَيْكُمْ مُرْسَلُونَ
We have indeed been sent to you. (Yā Sīn 36:14)
إِنَّا إِلَيْكُمْ لَمُرْسَلُونَ
We have certainly been sent to you. (Yā Sīn 36:16)
After the hearers reject the claim, a more forceful formulation is suitable. The underlying assertion stays the same; the linguistic strengthening reflects a changed rhetorical situation.