Line splitters are used to distribute/split an audio signal into 2 without noticeable loss. It may, e.g. be used for splitting a mono or stereo signal from a mixer to 2 different amplifiers.
Line combiners are used to combine 2 audio signals into one. E.g. if joining of 2 signals to 1 amplifier is desired, even without switching. One must of course remember that both signals will be active simultaneously, unless you dampen one while the other is active and vice versa.
It is always recommended to use an active combiner, as disturbances and direct defects may occur in simply connecting passively in a combiner/splitter cable. When the signal is split passively, i.e. with a Y-split cable, a sound reduction will often be noticed in the signal strength.