Maimonides claims that certain Mitsvot act as "protecting angels". One of them is the Mezuza. What do we need protection from? We need protection from forgetfulness. We live hectic lives. We are flooded with all kind of worries and distractions. Material needs that we need to solve urgently. Bills, jobs, tests, competition, etc. All these demands our attention NOW. We become so distracted occupying ourselves with thousand urgent matters, that we forget to re-focus on the most important thing in life: The purpose of our existence.
Obviously, meaning of one's life is only relevant for those of us who believe in God. If one believes that we are in this world by a cosmic accident that purposelessly produced life billions of years ago, he should not be worried about the question of "Purpose". Just goes ahead doing what he needs to do for survival. However, if you do believe that God created you, you know that He must have done it for a purpose! Living with that purpose in mind (or even for the quest of that purpose) is what gives meaning to our life.
Can you imagine a person who believes in God and yet is so absorbed in distractions that ends up forgetting what the purpose of his or her life was? That is spiritual suicide. The biggest human tragedy, perhaps.
The Mezuza is a protection against this type of existential forgetfulness. Every time we enter a room, we cross a door, or we go frenetically from one place to another, the Mezuza forces us to stop and remember our Creator. The Mezuza helps us to regain awareness of the bigger picture of life: what is really IMPORTANT and what is just URGENT.
Maimonides mentions explicitly this segula of the Mezuza. In MT, Mezuza 6:13 he says. "A person must show great care in [the observance of the mitsvah of] Mezuza. Whenever a person enters or leaves [a room], he encounters the name of God...and then remembers Him. Thus, he awakes from his sleep and his obsession with the vanities of time, and recognize that there is nothing which lasts for eternity except the knowledge of the Creator of the world. This will motivate him to regain full awareness [of his purpose in life] and follow the paths of the upright."
Maimonides calls the Mezuza (and the Tefilin and the Tsitsit) an angel of protection. Protection from forgetting our real mission in life. "These are the angels who will prevent a person from sinning, as it is written [Tehilim 34:8] "The angel of God is around those who fear Him, and protects them."