We use modal verbs in a situation where we need some level of deduction which means we say how sure we are about something.
Must
We use 'must' when we feel sure that something is true because we have some information about the situation, we have strong evidence.
She must live close to where she works because she walks to work. (the speaker doesn’t know where but is sure it is not far away)
You’ve been working in the garden all day. You must be tired.
Being a sky diving instructor must be very exciting.