It's Ritz-sensei's sense of humour and fanservice. The evidence has increasingly grown since the first season of the anime. The combination of yuri/lack of males and heterosexual relationships along with the emphasis on nobody in Saki wearing underwear makes for a very interesting universe.
In a lot of anime, showing underwear is considered a form of fanservice. In Saki, since there is no underwear, the line between what is fanservice and what isn't is blurred. By playing with her audience's expectations like that Ritz-sensei has been able to do some pretty incredible things.
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