The reason is very important in the sentence.
The students could go home earlier because the teacher was ill.
Because the teacher was ill, the students could go home earlier.
The reason is already known.
As we read, we learn.
The reason is already known. (more formal than as)
Since we were in the computer lab, our English has improved.
The reason is given at the end of the sentence. The clause with for cannot be used in inital position.
We went to a small restaurant – for we were hungry.