Durkheim says this because he believes that deviance is actually necessary for a society. If there are deviants within a societ, the society will be more able to pull together and its people will feel more solidarity with one another.
Durkheim says that deviant behavior forces us to be extremely clear on who we are as a society. When we define what behaviors are wrong, we can be more certain of what right behavior is. When there are deviants among us, it makes it easier to see where our moral boundaries are. We should also note that the ways in which we respond to deviant behavior draw us closer together as a society. We feel more of a bond with one another because we share the fact that we are not like the people who are deviant.
Because the presence of deviance makes it easier for us to come together as a stable society, it is important for us (Durkheim says) to have deviants.