You want her to grow up with respect for others, including the opposite sex. By placing her in an environment where she works closely with males on schoolwork, projects and extracurricular activities, you are giving her the chance to form a respect for them that she might not have attained through a single-sex school. She will learn how to interact with the opposite sex and how to understand the way they think and behave. It also teaches her to expect males to respect her as she grows older and this becomes something she wants from them.
If she attends an all-girls school, she will likely find herself less confident when she is around the opposite sex. Attending school with males will help her to gain confidence in herself and in her interactions with them. She will be able to speak easily with males, make friends with them and work together with them as needed. She will need these skills later in life when she is out of school and facing the real world.
There is a very slim possibility that after school your daughter will be segregated to a life of all-female counterparts. She will likely work with males, interact with males and encounter males during her down time while shopping or dating. Choosing to enroll her in a school where she learns how to do all of this now will be more realistic and will help prepare her for the world that awaits her.
Some people believe specific talents and interests are better suited for a certain sex. Seeing females in science and math classes and watching males take art and music courses will help each sex to know that stereotypes don't define a particular sex. Students who receive a co-ed education will quickly learn that someone of any gender can accomplish anything he wants to achieve.