differences between kna1,knb1 and knc1

Hi Gurus,

              I am new to sap abap. In tables kna1 , knb1, knc1  the tables are differed by some fields like burksetc.Could anyone explain me why not all the fields  should be in a single table. Why it was divided into tables like kna1, knb1, knc1.Please explain me in details what is the reason behind it.

Thanks in advance.