When something is incomplete and we don't understand it fully, we ask 'What's (use of) that..?" (tatah kim..?)

Guru ashtakam says, without attaching our 'manas' to the feet of 'Guru', nothing is complete. So we ask 'What 's (use of) that..?'

Guru gita says Guru is the 'Atman' that controls the buddhi. Atman is the 'detached observer' of this Universe, that guides the evolution of our Universe, just by observation or just by being a witness or sAkshi. This Observer Atman manifests in beings like us as the 'Observer' of our 'Manas'.

Manas are thoughts that control our actions. The observer of our Manas or the 'Witness' of Manas is Manas-sAkshi. This observer in us is the 'Guru'.

We call an external person a 'Guru', when that person just by the presence, guides us and evolves us, purifying our defects. So Guru Ashtakam says, without being guided by this observer (internal manas-sAksi or external guru), whatever we do is incomplete and hence asks 'what's that..?

Guru Poornima is celebrated on Ashada poornima because the month of Ashada is the start of daksinayanam, the start of journey of Earth to darker, colder times. Guru is needed most in these dark times. Hence Ashada month is chosen. The day of poornima is choosen, because Chandra/Moon represents mind and full-moon represents being guided by observer with full mind.

[embedyt] https://www.youtube.com/watch?v=3RTdkXt9oCk[/embedyt]