the reason for column marks (why do ancient doric columns have flutes?)

an example of doric columnata 

Why do we need columns? 

In architecture, a column's primary job is to act as a vertical structural element that supports the weight of a building by transferring compressive loads from the ceiling or beams above to the foundation below, essentially holding up the structure and allowing for larger open spaces within a building; while also serving as a decorative element in many designs.

Take a look at this example of Doric columns. They are shallow and always end in a sharp ridge, unlike the Ionic flutes that always had a narrow, even plane between the flutes (called fillets).

Why the fluting?

1- they add dynamics to the column by emphasizing both verticality and roundness
2- they add a counterpoint between the flutes and the round form of the column (this is the ceremonial look to the temple)

See the difference between a flute and no flute.

Tempietto by Bramante

The deal is that this is a marble column. No better or worse, just different. 

This is Renaissance. Bramante is doing something different than the Greeks & what would that be?