If the values 1 and 4 were removed from the cells I4 and I5, the Sudoku would exhibit a Bivalue Universal Grave pattern (or BUG).
In a BUG, each value that remains in a row, column or box has exactly two possible positions in that row, column or box; and each empty cell has exactly two potential values. A Sudoku having a BUG has zero, two or more valid solutions.
Because a valid Sudoku has exactly one solution, the BUG cannot be part of it. The only possible way to avoid the BUG is if one of the two cells I4 or I5 contains one of the values 1 or 4. If follows that the other of these two cells will contain the value 2 because this value has no other possible position in the box 6.
The values 1 and 4 can therefore be removed from the cells I4 and I5.