Eikev - Shishi
Rain, Rain, Don’t Go Away
Egypt, which was the people's only frame of reference, was irrigated by the Nile. Moshe informed the people that Israel was not that way. Israel is full of mountains and valleys that rely upon rain. The land requires God's special attention to ensure that its agricultural needs are met. If the people are faithful to God and serve Him whole-heartedly, He will send all the rains at their appropriate times and the land will yield a bountiful harvest. If the people stray, God will withhold the rains, there won't be any crops and the people will disappear from the land.
These words were also to be included in tefillin and mezuzos. (Verses 13-21 constitute the second paragraph of the Shema.)