It's a Mitzvas Assay (positive commandment) to put a MEZUZAH on the doorpost.

And we have to be very careful to do so. As our Rabbis say, that in the merit of the MEZUZAH, you and your children will be blessed with long life.

G-D will protect your home, like the story the Talmud brings about Onkelus, that the king of Rome sent for him, and he told them the greatness of the God of the Jews, namely that a king of flesh and blood sits inside his palace and his guards protect him from the outside, but the God of the Jews is different; His servants are inside while He protects them from the outside.

There's a story told by Rav Yosef Karo (author of the Shulchan Aruch) about a city that had a great sofer (scribe) who was very pure and holy. Once there was a fire in the city, and every house that had a MEZUZAH written by this sofer (scribe) remained untouched by the fire.

The stories about miracles and wonders that happened through the MEZUZAH are endless. This divine protection is granted only if the MEZUZAH is written properly. Otherwise, not only does it not do any good, but it may, God forbid, bring danger and damage to the one who uses it.