"Draw random colored rectangles over the redrawn areas" understanding

While using v8.2.1, I created an empty screen

I noticed while debugging the redraw areas, it draws areas depending on the lv_disp_draw_buf_init buffers size:


I was expecting just one full single rectangle.

Is this supposed? does it has some meaning on relation to widget redraw?
How does the buffer size relates with the widget redraw updates?
Are there benefits from choosing a small or larger buffer?