The verb to be is very useful!
It can be used to mean 'to exist':
To be or not to be: that is the question. To live or not to live: that is the question.
I think, therefore I am. I exist because I think (Descartes).
As well as being an auxiliary verb it can express:
• The progressive form, if it is followed by a present participle (-ing form) :
He is sleeping. He is asleep at the moment.
They were talking when Bruno arrived. Bruno arrived at a point in time when they were talking.
• The passive voice, if it is followed by a past participle:
The table is set. Someone has set the table.
The car was repaired by my brother-in-law. My brother-in-law repaired the car.
• Obligation, duty, agreed actions (in the future), if it is followed by an infinitive:
You are to do what I tell you to do. You must and will do what I tell you to do.
The Prime Minister is to arrive this afternoon in Paris. The Prime Minister should arrive in Paris this afternoon.
The meeting is to take place in the boardroom. The meeting will take place in the boardroom.
To be also forms expressions made up of auxiliary verbs which express various modalities: be likely to (probability), be sure to (certainty), be going to (future), etc.