This is part of a series on Korbanot, an oft-forgotten topic that deserves more attention. Be sure to check every day for a new installment!
רבי יהושע בן לוי אמר תפלות כנגד תמידין תקנום
Rabbi Joshua the son of Levi said, "The prayers were instituted parallel to the Constant Offerings.
There is, however, a problem with this statement.
There are only two Korbanot Tamid, One in the morning and one in the afternoon. Why do we have the evening prayer as a substitute for a Korban?
And why did they say, "The evening prayer: it has no set time? Because the sacrificial limbs that have not been consumed by evening are burnt, and continue all night.
Back then, the evening service was mainly to finish the leftovers that, so to speak, fell through the cracks. So too with our Tefillat Ma'ariv. Although only Shacharit and Minchah are 100% Commandment, Ma'ariv is still very much required, because no one is perfect and we all, naturally, fail at least in the slightest during prayer.