It is used to express---
a. A moral duty or obligation.
b. To express a strong possibility.
Note: Ought indicates present tense when followed by the present infinitive. But it indicates past tense when it is followed by the past infinitive.
a. Friends ought to help each other.
b. Our team ought to win the match.
c. He ought to do it.(present)
d. He ought to have done it.(past)