Why are there different sections for initialized and uninitialized globals?

Question

I'm reading this course on the structure of an executable and it says there are three data sections in an executable:

• code section — where the instructions are stored
• data section
  • .data — stores initialized global data
  • .bss — stores uninitialized global data
  • .rodata — stores read-only data, such as literals

My question is, why is the distinction made between initialized and uninitialized global data?

(We use C in class, but I guess this is a language agnostic subject.)

Answer 1

Image size. The program image has to contain the initialization data for .data, but it does not have to contain .bss.