Why would we wash semen?  The short answer is that if we had a lovely collection we wouldn't!  However to optimize a bitch's chances of conceiving and to protect her from possible infection then semen washing may be required.

For example, a contaminated sample can be toxic to the sperm in the collection and put the health of the girl and her litter at risk if it is placed in the girl untreated.

This is one of the many times that using a great clinic and working with someone who truly understands canine fertility can make all the difference.