Control of the mind is the aim of yoga
...regulation of breath is prescribed for making the mind quiescent. Quiescence lasts only so long as the breath is controlled. So it is transient. The goal is clearly not pranayama. It extends on to pratyahara, dharana, dhyana and samadhi. Those stages deal with the control of mind. Such control becomes easier for the man who had earlier practised pranayama. Pranayama leads him to the higher stages involving control of mind. Therefore control of mind is the aim of yoga also.
A more advanced man will naturally go direct to control of mind without wasting his time in practising control of breath. A simple development of pranayama alone may confer siddhis which so many hanker for.
(Ramana Maharshi in Talk 154, 5 February 1936)