If we use the subjunctive form we are using a more polite form than in the indicative form. Look at these two sentences:
Indicative: Lei ha sbagliato.
Subjunctive: Penso che Lei abbia sbagliato.
So if we want to talk in a polite, respectful way we should use the subjunctive form.
See also:
Italian Course: Unit 21 The present subjunctive tense
Italian Course: Unit 23 The imperfect subjunctive tense
Italian Course: Unit 25 The pluperfect subjunctive
Italian Course: Unit 28 The present perfect subjunctive tense
Italian Verb Tenses Exercises Imperfect subjunctive, Past subjunctive, ...
