Why do you use chemicals?

Chemicals are anything that are made of matter, which includes all solids, liquids and gases - and therefore the human body is made up of chemicals. However, if you are referring to synthetic chemicals, that is those that are manufactured by man, then they are used in every modern application you can name. The main industries are the petrochemical (fuels, lubricants, etc), agrochemical (pesticides etc) and pharmaceutical (medicines), to name but a few.