Ralph Fedorak writes:
First what is considered thin?

It depends on the style of the object and its size. On a 12" bowl,
1/4" would be "thin" for me, with 1/2 to 3/4 being "thick". It also
depends on the treatment you give the rim (decorations, bands, etc).

Second,why should walls be thin?

Because thick walls make the bowl look chunky and unappealing.

For that matter why should bowls be thin?

Because if they're too heavy they're unwieldy. And ugly.