The purpose of the light was to mark time, the revolving of the earth. To wit,
Genesis 1:5 God called the light "day," and the darkness he called "night." And there was evening, and there was morning--the first day.
Unfortunately, you eventually "telegraph" your real motive for your circular, sola scriptura, one-sided reasoning in your absurd #52:
"The purpose of the light was to mark time, the revolving of the earth. To wit,"
There is zero relationship (either scriptural or scientific) between God's initial, omnidirectional but anisotropic, release of photons -- and the human convention of marking the angular position of light rays from the (later-formed, self-iluminated) sun, striking the "revolving" earth -- as a means of earthly time measurement.
The inevitable follow-on claim to your position edicts that the duration of the days ["yom"] of our eternal Creator God are dictated by the spin rate He imparted on this [created, then (later) formed] ball of dirt -- three Divine days after He caused light to be released from the "stuff" or "dark fire" of His original, simultaneous, instant creation of energy, time, matter, and space.
IOW, your statement is a vain, human scheme to dwindle almighty God down to human-comprehensible scope. There is nothing righteous, holy or worshipful in it; quite the opposite.
And, most certainly, there is no such "purpose" for "Let there be light", -- either stated or implied -- in Genesis.
Your above statement is a purely human-contrived vanity. I defy you to support it with Scripture. And I challenge you to relate it to the actual reality of God's created universe (which you scornfully ignore).
You would be well served to consider prayerfully this truth often stated by Alamo-Girl:
"Man is not the measure of God."