Definition 1 of 1
In this case the function 'on' means 'to do' or 'to use', as in sitting to use the computer, or using the computer.
For reasons unknown to us it is important to Indians to specify that this activity is carried out while sitting down (and not while squatting, because that would have unsavoury connotations).
Limbu: "I don't feel like cleaning my room."
Mum: "Ok then study."