DJ is not supposed to have the record. In some cases a track has been promised to another A-list DJ for a compilation exclusively, this means the dj will want to keep it very secret to keep it a suprise of the CD tracklist release. If he/she finds out the song is being played by another dj on a radio shows he/she may not choose it for the CD compilation in favour of something that has yet to be played by anyone. By playing it as ID the dj can hide the fact that the track is being played, this also shows respect for the DJ who is paying for it to be on his CD and allows him/her to be the one to announce the name, this keeps his radio audience happy with fresh new tunes and the DJ making the compilation still uses it for his tracklist. Win/Win.