The reason as I understand it is to do all we can to protect the Holy Body and precious Blood we have received in Holy Communion. The idea being that if I brush my teeth and spit out the yukky water, I may also be spitting out some small fragments of my Lord's Body that may have remained lodged between my teeth. Similarly, if I have a shower and some soapy water goes in my mouth, I am likely to spit it out pretty quick.