WHAT IS THE MAIN PURPOSE OF VEDANGA JYOTISHA?

Vedas are intended for the performance of yajnas.

VedAnga Jyotisha is also known as KAla vidhAna SAstram as understanding time is necessary for performance of yajnAs.

vedA hi yajnArtham abhipravruttAh
kAlAnupUrvyA vihitAsca yajnAh !
tasmAd idam kAlavidhAna SAstram
Yo jyotisham veda sa veda yajnAn !!