Chemical tests such as precipitates and flame tests rely on a person to judge a colour by eye, and they need a fairly large amount of sample. Instrumental methods use machines to analyse a sample. Flame emission spectroscopy is one example, and it identifies metal ions by measuring the light given out by a sample in a flame.
Instrumental methods have three main advantages. They are sensitive, which means they can detect a tiny amount of a substance in a small sample. The word for this is sensitivity. They are accurate, because a machine measures the result and does not depend on a person judging a colour. They are rapid, so results are available quickly and many samples can be tested in a short time. This is their speed.
These advantages matter when a sample is very small or a pollutant is present at very low levels, for example when testing water or checking evidence in a forensic laboratory. The disadvantages are that the machines are expensive and the operator needs training.