Toothache. The purpose of this medicine?

Hello sir.. Usually when you have pain and infection on your teeth then root canal can be performed in 2 or three sittings in order to reduce the infection.They did the first step of canal opening...and the medication in the cotton they kept inside the canal is to kill the infection causing bacteria...you need to go for Analgesics and antibiotics course for 3 days for better recovery...In the next visit they remove the cotton inside the canal and check for cleanliness inside the canal whether it is dry or not..If it is dry then go for further steps of canal reshaping and canal filling with Gutta percha.....If the canal is still weeping(WET) then cotton packed with medication is still continued for 3 more days until the canal gets dry

in first sitting the canal is cleaned  after devitalising the nerves.Then the medicine is placed with cotton .In your next appointment , cotton will be removed and will fill the canal by medication . Should continue medicine atleast for 3 days.

if it was RCT as mentioned by you , then the first step has been achieved , canal opening , second step would b preparing n cleaning the canals, finally restoring it . coming to your question, what is the purpose of cotton ,/ it contains medicament which helps in killing the infection.