Why is it important to have laws that stop discrimination?

If a society or a company didn't have laws/rules against discrimination, that society would probably have more people actively discriminating against people.  Not everybody is going to altruistically avoid discrimination.  Having laws against it shows people that there are rules about certain kinds of behavior, and if those rules are broken, then punishments will be issued.  In general, people want to avoid punishment, so rules against discrimination work to encourage people to not discriminate.  Consequently, they avoid punishment.  Additionally, if a work place is unable to discriminate (for whatever reason),  that work place likely becomes a place with more diversity.  That is generally a good thing, because increased diversity allows for more ideas and a wider range of creative thinking.  It is also likely to breed an environment of tolerance.  As people in one environment learn to tolerate diversity, they are likely to extend those skills into other areas of their lives, too.  Finally, laws that work against discrimination also work to protect the people that may be subject to that discrimination.  The laws help put them on an equal footing with everybody else.  
https://www.referenceforbusiness.com/small/Qu-Sm/Racial-Discrimination.html