Shortly ...
The last is something that might seem strange to a lot of people, whether you've been a church-going Christian for a million years *cues drums for bad joke* or you've never set foot in a church with a cross on it in you entire life. There are a some groups of Christians for whom the singing part of a meeting is mostly about singing, and that singing can be about God, about other God-followers, to other God-followers; this sort of Christian gathering tends to leave the lights on, for a variety reasons, some as practical as being able to read the hymnal. Other groups of Christians, however, believe that the singing part of a gathering, often specified as 'worship' in such groups, believe that singing should primarily be to God rather than about him, about others, to others, etc. Christianese often terms this 'intimate worship'. The idea being that we as humans were designed for a love-based relationship with our creator and there is a recognition that one powerful expression of that love-relationship is through song. And here there is a dichotomy of sorts: there is something special about outwardly expressing love for creator/savior/lord/friend in a corporate setting, and conversely, this expression is most intimate when the expression is between just one worshiper and her/his God. The middle ground sort of solution that some churches have come up with for being able to encourage individuals to worship intimately in a corporate setting is to get everybody together to sing and pray to their God in one-on-one language and turn off the lights so that it is that much easier for individuals to focus on God, rather than on each other as one might tend to do if the lights were on.