Why rated voltage is 415v?

Well, first it's a question about deciding on a specific voltage in the first place. The specific value it's not really that important, but suppliers and makers of electric equipment has to agree on a common target to build their products for.

Like deciding which side of the road to drive on.

Next thing is the issue of 3-phase distribution.

The concept of 3-phase, which is a neat way of building generators, motors and distribution equipment, means that you'll get a certain ratio between the voltages available.