Why is line emission spectrum so special?
Probably because it gives us an insight into the internal structure of an atom.
As a simple example we can use the hydrogen atom where the emission spectrum allows us to "see" its internal structure of allowed energy levels. The spectrum is like a map that shows us the disposition of the energy levels of the atom that otherwise would be quite difficult to observe.
We can also think that hydrogen spectrum was one of the first indication of the existence of some quantized structure in the atom and so the first step into the development of Quantum Physics.