It is usually said to be worn because the head must be covered in the presence of God. This leads to some marked variations in use. After all He is everywhere. Some men wear a kepah/yarmulke all the time others only when attending the synagogue or when taking an oath in court. It largely depends how 'correct' the individual is and which branch of judaism ( i.e whether Liberal , Orthodox or whatever)he belongs to.
It causes some debate. There have been Jews who wondered about wearing one in Alcoholics Anonymous meetings, for example ( the organisation is supposed to be non-religious ).
It is a conspicuous sign of the wearer's faith. Some Jews evidently feel it unnecessary to show this all the time in public. It is tempting to say that for them it is like a Christian wearing a cross pendant, but it is much more significant than that.
