By HaRav Shaul Yisraeli, zt"l
In the context of the oil used for the Mishkan, the Torah writes, "tzav (command)." Chazal tell us that this implies a commandment that was to be carried out immediately but was also to apply for generations to come and that it is also a term that is used when there is a need to deal with an outlay of money (Sifrei, Bamidbar 1).
The connection between Hashem and His Torah and Bnei Yisrael is a natural one. "I will never forget Your instructions, for through them You have given me life" (Tehillim 119:93). They are natural for us. As the Mahari Mintz says on the pasuk, "For it is not an empty matter for you," it is not something that can be emptied out of you.
This is the idea behind the commandment for now and for future generations. This is a sign that it is not something external, but something that flows from our essence. The nations of the world complained to Hashem over the fact that He did not hold Mt. Sinai over their head and force them to accept the Torah, as he did for Bnei Yisrael (Avoda Zara 2b). The Yalkut (Bamidbar 684) adds to the midrashic account that Hashem responded that they should show Him their genealogical records. How does that respond to their complaint? Hashem was telling them that based on their predecessors, it would have not helped to hold the mountain over their heads because they were not linked to the Torah in a natural way.
This also explains the statement of Chazal (Sifrei, ibid.) that tzav implies rallying the people to be diligent, which is effective specifically for those who are naturally diligent. In order for it to work, people need to have a natural proclivity to get the job done. That is why the commandment will be fulfilled even when it requires a loss of money.
"Now, command Bnei Yisrael" (Shemot 27:20). Even though they are Bnei Yisrael, their completeness will express itself only if you command them. If they just do a good deed because their emotional feelings bring them to it, then there will be a lack of longevity and consistency to the good deeds. We know and see how Jews who do not conform to the obligation to follow the Torah often display "a Jewish good heart." This is because they come from a chain of generations of people who kept the Torah. However, only when there is a commandment for now and for generations can we be sure that mitzvot will be kept consistently and not just when the mood or ‘the weather’ points in that direction. By being something that starts immediately and continues forever, a connection is created between the distant past and the distant future.
These ideas are also engendered in the pasuk, "You are they who cling to Hashem, your G-d, you are all living today" (Devarim 4:4). Just like the day brings light to the world, which is a natural thing, so too the clinging of Bnei Yisrael to Hashem is natural and brings light to the world.