The  main  advantage  of  binding  with  plastic  comb  elements  is  the  possibility  of  adding  or  subtracting  sheets  without  damaging  the  sheets  or  the  plastic  comb  elements.  
When  binding  with  plastic  comb  elements,  the  pages  can't  be  turned  by  360  degrees  as  spine  gets  in  the  way.
Plastic  binding  combs  are  suitable  for  binding  documents  up  to  48  mm  thickness.
Round  plastic  comb  elements  allow  pages  to  be  turned  easily  and  to  be  laid  flat.
Oval  plastic  comb  elements  designed  for  thicker  books  for  more  strength.

Check  binding  thickness  table  in  attached  PDF