Pre cleaning

Why do we brush our teeth before going to dentist for a 'cleaning' appointment? I mean what's the point. The dentist is one person that sees all that filth in our mouth and still manages to keep a straight face. The reason they wear a mask is ofcourse to hide that ghoulish look on his face. I for one reason brushed my teeth before my appointment today -I was seeing a new female dentist. And as plaque-ful my first appointment could have been, I still had to put my best teeth forward.