Well,really it's usually called the magwell dimple(s) , but it's original intent was to help keep the magazine centered in the magazine well.
Can an AK work without the dimples? Sure,it just adds a little more stability.
On a "normal" AK the lower rails inside of the receiver,do not extend into the area where the dimples are so the dimples center the magazine.
In an AK such as a WASR the bottoms of the lower rails have an extension on the bottom of the lower rails that center the magazine in the magazine well.
The Saiga doesn't have the dimples, I believe as a purely cosmetic difference between the Saiga receiver and a "normal" AK. This is, I suppose to make it "not an AK receiver" and somehow miraclessly politically correct.
When it comes right down to it the lack of dimples has no effect other than disturbing the "look" of the effected AK.