Parashas Eikev · Passuk 8:16
who fed you in the wilderness with manna, which your ancestors had never known, in order to test you by hardships only to benefit you in the end—
Torah Temimah points to Yoma 74b.
TT lemma: למען ענתך.
English: What is “in order to afflict you”? R’ Ammi and R’ Assi: one said, one who has bread in his basket is not like one who has none; the other said, one who sees and eats is not like one who does not see and eats. The manna had both lacks: it fell only for the day, and they saw only manna though it tasted of many foods. From here a hint that the blind eat and are not satisfied — and midrashically a hint for Shabbat candles: since eating without seeing is called affliction, and Shabbat requires oneg, one lights a lamp to see one’s food.
Custom (affliction of the manna; hint for Shabbat lights)
The manna’s daily uncertainty and lack of visible variety are forms of affliction; seeing one’s food is part of oneg Shabbat.