Simple chemical tests rely on a person seeing a colour change, a precipitate or fizzing. Many substances are now analysed with instrumental methods, which use machines. A flame photometer, used to find metal ions, is one example.
Instrumental methods are more sensitive. This means they can detect very small amounts of a substance, such as traces of lead in drinking water, at very low concentrations. A chemical test needs enough substance to produce a colour change or precipitate that you can see, and a small sample may be lost or give no visible result.
They also have greater accuracy, because the machine gives a measured value instead of a person judging a colour by eye. They are faster as well, with greater speed of analysis, because they are automated and can analyse many samples quickly using only a very small sample.
The drawbacks are that the machines are expensive and the user needs training, and the results often have to be compared with reference data before they mean anything.