Why do we excite electrons before hybridisation?
Actually number of bonds formed by an atom is generally equal to number of unpaired electrons it has. Like oxygen and nitrogen make two and three bonds because they have these many unpaired electrons in their valence shell. 
But, in its ground state, carbon got only two unpaired electrons, so it should make two bonds. But in reality carbon make four bonds. Two satisfy four bonds we need four unpaired electrons. This is possible to consider the excited state.
Hope its clear.