What's the use of static batching?

Hi there,

The manual says: If several objects shared the same geometry before static batching, then a copy of geometry will be created for each object, either in the Editor or at runtime. What sense to create copies at build time increasing app size? I can do the same combining copies in one big mesh in my 3d editor. What sense? But manual says static batching can be performed at runtime? How is this possible? How to achieve that.

Nobody can give clear answer during two days.

Thanks for help.