Connects to 'line-in'!
Quite simply, 'line out' provides a matching input for, say, hi-fi systems which have a line-in socket. Loudspeakers expect an impedence of 4 - 8 ohms; line-in terminals 600 ohms as I remember. If you have a mismatch you might well have a distorted input - apart from the possibility of blowing your input circuits if you put a lot of power into an amp which isn't expecting it.