The need for a deity is for one to have focussed and meditative attention- Ekagratha.
The Tamizh word Kadavul -God is Kadai- meaning Body; Ul- meaning inside or inner-self.
So the never-changing inner truth within the maya or illusionary ever-changing body form is Kadavul or God.

So in a temple, one needs to identify with the deity and discover the godliness within oneself.
The power of the deity is the power of your belief.