حَضَرَ الْمُعَلِّمُ.
The teacher arrived.
إِنَّ الْمُعَلِّمَ لَقَدْ حَضَرَ.
Indeed, the teacher has certainly arrived.
The first form supplies ordinary information. The second intensifies commitment through inna and the emphatic lām. For a listener who has no doubt, that level of emphasis may be excessive; surprise or a prior challenge could make it appropriate.
The lesson is not that stronger grammar is always better rhetoric. The degree of reinforcement must fit the addressee's state and the speaker's immediate purpose.