Question

Why is it important that each cuvette is inserted into spectrophotometer in the same way(with the arrow on the cuvette facing forward)?

Answer 1

The cuvettes for spectrophotometers are specially designed for each model, they generally allow the passage of light only on two sides (or in other cases, the passage of light is more precise by a specific pair of faces), additionally the direction of the flow of Light is important for an accurate measurement, which is favored by the design of the cuvette to receive the light beams in said direction and by the pair of faces mentioned above.

These are the main reasons why you always have to insert the bucket in the direction designed by the manufacturer.