A single proposition can be expressed with different levels of emphasis. The following are teaching applications in addition to the particular source example.
زَيْدٌ فِي الدَّارِ — Zayd is in the house; a plain assertion.
إِنَّ زَيْدًا فِي الدَّارِ — Indeed, Zayd is in the house; strengthened assertion.
إِنَّ زَيْدًا لَفِي الدَّارِ — Zayd certainly is in the house; further emphasis.
For a hearer with no prior view, the unmarked statement may suffice. In the face of denial, added confirmation may be apt. These are context-sensitive choices rather than fixed formulas independent of purpose and discourse.