A jury is the last line of defense. The state can not use it's power to punish someone unless they can find 12 members of the community who are willing to allow the punishment.

It is supposed to be the last, final defense against the perversion of the justice system into political prosecution.

The state can make whatever laws it wants, but it can't enforce them without the will of (some of) the people.

Probably worked, too, when populations were smaller and communities more cohesive.