The vādi and the samvādi notes are emphasised constantly and are used as a point of reference, so as to enable the listener to recognise the raga and rasa (mood).  All other notes in the Raga are called anuvādi, and the vivādi note is a dissonant note  which is said to destroy the Rasa (or mood) of the Raga.  So, therefore, it is only used for special effects, but it can be used in some Ragas to beautify the music.  

The art of Indian music can be called guided or disciplined improvisation, because the improvisation has to be in accordance with the prescribed note patterns of the raga.  If these rules are not adhered to, the raga could be confused with another raga of the same thaat.