Prevent iron loss. If it is a whole piece, the flux linkage will produce eddy current, and the eddy current will cause energy loss. As for why it is a silicon steel material, it is because silicon steel has a higher magnetic permeability and a narrower hysteresis loop. The magnetic material is not easy to saturate, and the volume of the transformer with the same rated power can be made smaller.
The silicon steel sheets are arranged and compacted and welded into an iron core. There will be a very narrow air gap between the silicon steel sheets. In actual production, a vacuum bath is used to immerse the insulating varnish, so that when the magnetic path is formed, it will not be like a whole iron core. Eddy current is generated and iron loss is small. At the same time, the small insulation gap will not produce large magnetic loss.
The shape is divided into E piece and I piece. The coil is set in the middle of the E piece, and the I piece is buckled and welded (only in general, the actual process is complicated).