There are many benefits to be gained by using a double beam spectrophotometer with matched cuvettes.

These are selected to have identical optical properties, so one can be filled with the blank and left in the reference beam while samples are measured against it, saving time and improving accuracy.

However ,as the blank is left in the sample holder, while the other one is inserted and removed many times and is also subject to the effects of interfacing with your samples, one will age much quicker than the other.

One way round this is to swap them over on alternate batches of samples to even out the impact of wear.

Of course if one of them gets a scratch or stain on an optical surface they are then effectively unmatched, and would need to be cleaned or replaced.

To check the condition of your cuvettes run a baseline with nothing in the cell holders, the put one cuvette in the sample beam and the other in the reference, note the reading, and then swap them around and note the new reading.

The difference between the two is the possible error in your results, and if this is unacceptable, the cuvettes need to cleaned or replaced.