Why is a circular groove provided in saucer for placing the tea cup it?

The circular groove in a saucer is not primarily for placing the teacup. It's actually there for a different reason:

To prevent the saucer from sliding on the table.

The groove acts as a lip or rim, creating a slight indentation that helps the saucer grip the tabletop surface. This makes the saucer more stable and less likely to slide around when you pick up or set down the teacup.

The shape of the groove itself doesn't directly affect the placement of the teacup. The teacup sits in the center of the saucer, and the groove is simply a design element to ensure stability.

Think of it like the ridges on a tire - they provide grip and prevent the tire from slipping. The groove in the saucer functions similarly by providing a friction point with the table surface.