The purpose is to obtain the correct physical configuration of Śruti swara arrangement to Shadja Grama Notes on a veena instrument (Sa, Ri, Ga, Ma, Pa, Dha, Ni corresponding to 4-3-2-4-4-3-2 totalling 22 Srutis in a Saptak).
The experiment is described in Abhinavabharati, a commentary to Natya Shastra, as an explanation after verse 28.26 of Natyashastra.
This assumption though not explicitly stated in the Natya Shastra needs to be made as was discovered by Avinash Balkrishna Patwardhan and is required to prove the Sarana Chatushtai experiment.
Setting the Chala sitar in the Madhyamagrama by lowering the Pa note by one Pramana Shruti is the first part of the first Sarana.
Only when all 4 sarana’s are successfully tuned and the sound notes cross checked as mentioned by Bharata, then only one can know the 22 shrutis of Shadjagrama as required by the Natya Shastra.