7. In a nutshell, to prevent public school teachers from being arbitrarily fired.
Tenure is part of collective bargaining. Without a guarantee of due process, a teacher can be fired for any number of reasons. It won't be called firing but rather, that teacher won't be hired for, say, another two-year term.
Name any reason: Students get together and decide on a campaign to remove a teacher who gives low grades and too much homework; a parent decides s/he doesn't like that teacher (something personal, teacher teaches evolution and parents wants "intelligent design" in the curriculum), principal wants that teacher to raise a grade for a particular student; teacher assigns a controversial book like "Catcher In The Rye".
And teaching is a career, not a job. Where is a teacher supposed to go to work after "not being rehired"? Work is not exactly available everywhere. A teacher and his/her spouse may have settled in a community and don't want to move. They certainly can't move and buy/sell homes every two years.