Time-bound mitzvot
The Sages established (Kiddushin 33a; Rambam, Laws of Forbidden Relations 12:17): "Women are exempt from all positive, time-bound commandments" - positive commandments dependent on a specific time (such as sukkah, tefillin, shofar). Mitzvot not bound to time (such as mezuzah, tzedakah, kindness) they are obligated in.
The land's mitzvot and time
The land-dependent mitzvot are divided in two. Some mitzvot are also bound to time, such as terumot and ma'asrot that apply every year, shemitah in the seventh year (Vayikra 25:1-7), and orlah in the first three years (Vayikra 19:23). And some mitzvot are not bound to time but to place, such as kilayim of the vineyard and kilayim of seeds that apply at all times.
Settling the land
The Ramban (Additions to the Book of Commandments, positive 4) counts the mitzvah of settling the land as a mitzvah in itself, and the land's mitzvot are tied to the holiness of the land and the special obligation of its inhabitants. A Diaspora Jew, who cannot perform these mitzvot in his body, needs agency and partnership.
The greatness of the merit
One who partners in the land's mitzvot merits not only positive commandments but also time-bound mitzvot, which are anyway difficult to fulfill even for inhabitants of the land due to dependence on time and field. The emissaries in the field act at the proper time - at the harvest, at the reaping, in the sabbatical year - and the partner takes on the cost and is considered as if he performed it himself at its time.