Which is correct, “hospital” or “the hospital”?

Which is correct, “hospital” or “the hospital”? I've heard "He is going to the hospital." and "he is going to hospital." I've often wondered if one is incorrect.

There are certain buildings whose primary function varies. For example, in a hospital, people go for treatment. If these buildings are used for their primary function, in that case, ‘the’ is ‘’
omitted.

But if the speaker visits the place for not the primary function, the article ‘the’ is used.

Example:

Last night I visited a hospital. ( Here's the visit is for treatment.)

Last night I visited the hospital.

(Here, the visit is meant for seeing a patient or any other function than medical treatment.

I go to office daily at 9 a.m.

I go to school daily.