There are a few ways of expressing the future, besides the future tenses.

When there is no human control:

It is going to rain

..we cannot use 'be to'.

It is used to show human control over the situation.

Formal arrangements

The directors are to meet at the head office tomorrow.

Formal instructions/appointments

Active: You are to deliver these by tomorrow at the latest.

Passive: These are to be delivered by tomorrow at the latest.

Prohibition

These tablets are not to be taken.