In short, Shadjamam and Panchamam notes are special because they act as epicenters, around which other swaras dwell. In this post, I would like to bring some interesting aspect, what I call "Sa Pa Symmetry". Play a sequence/pattern of notes with your Sa as the home key and try to play the same sequence/pattern of notes with lower/upper Panchamam as the home key. There you go!
Keeravani and Mayamalavagowli raaga offered this symmetry naturally. Raaja has used this pattern in many many songs. To highlight with a classic example, "Poovama Oorvolam" song from Chinna Thambi. Listen to the verses -
"Aranmana Annakili Tharayile Nadapadhu Nadakkuma Adukkuma
Paniyilum Vetta Veli Veyilum Ulla Sogam Aranmana Kudukkuma"
If the first line of the above stanza is centered around upper Shadjamam, the second one follows with lower Panchamam. Another example in "Poova Eduthu Oru Maala" from "Amman Kovil Kizhakkale". Listen to the verses -
Other evident usages that comes to my mind immediately are - "Om Namaha" from Idhayathai Thirudadhe; "Poo Poo Poo Pootha Solai" and many more..
This might be one of the reason why Keeravani and Mayamalavagowli are very popular in light music and especially Raaja has used to its full potential in film songs.