What is the comparison to rain and snow?
Rashi: They do not return empty, rather, they improve the world.
Malbim: Snow satiates the land and makes it proper to give birth, and gives seed. Rain makes vegetation sprout, and gives bread 1 to eat. These are only for those prepared to reap their benefit - v'Nasan Zera only la'Zore'a (one who seeded), and Lechem only to one who prepared himself to [harvest and] eat. Likewise, every influence from Shamayim is uniform; the only difference is between people's preparation and ability to receive it.
Both seeds and bread come from the grain that grows! Perhaps Malbim means that if there was only snow, the land would produce only what is needed for seeding (the amount that was planted). Via rain, it produces more, so one can eat bread and still have seed for the next year. (PF)
What gives seed to the one who seeds?
Radak: The rain does. This is like "v'Nasnah ha'Aretz Yevulah" (Vayikra 26:4). From the land's yield, first man stores away what he needs to seed the next year, and the rest is to eat.
Does the rain return to Shamayim, and what is the significance of this?
Radak citing his father: The sun's heat causes it to return. If not for this, the land would not need rain!
Malbim: It does not return, for Shamayim does not need it; it is only for the land. This answers two claims why Teshuvah cannot help. (a) Regret does not fix what was ruined. This applies when a slave or worker works for his master's benefit, and he ruined the job. Hashem has no need for our Mitzvos. They are to perfect man! (b) Hashem does not change. He always influences good; people can receive it based on their preparation. E.g. one who did not seed his field, rain will not help him.
