Why again funtion modules in ABAP OO ?

if you look at the code inside

cl_gui_frontend_services->gui_upload they use gui_upload function.

there is no difference.

internally the class also uses GUI_UPLOAD ..........

there is conflict like data consistancy problem with function module ..

why again sap is using funtion modules in class method ...

Instead sap can develope method purely by coding in method know ?

Can any body explain what are the advantages and disadvantages ?