That one is fairly straightforward. In general, however, there is little point in asking why we use certain prepositions in certain phrases. Prepositions are a law unto themselves - in most languages that have them. We watch things on television, the French watch things at the television, and Germans watch things in the television. That's just the way it is.
Context is important. Please provide enough for us to be able to deal effectively with your question.
Your thread title should include all or part of the word/phrase being discussed.
If you just want to know the meaning of a word, try OneLook Dictionary Search first.