Question

where can we use must have

Answer 1

we should use it when something should be done

Answer 2

Thanks for your question !

We can use 'must + have' in such sentence which show less possibilities of things to be done.

Let me explain with - For example,
She must have to write a letter to her mother. Here, must have shows the less chances of letter to be written.

Somewhere, the structure 'must + have' obviously need the joint of 'to'. So we can express the same in form of 'must + have + to'.