Since a long time intelligent philosophers wanted to know the structure of the substances in the world, about the universe, matter, planets, air , water etc.
Investigations into the fundamental particles that are present in different types of substances and elements were done by all scientists or philosophers from many countries. It was to be able to know what kind of structure gives rise to what properties and then to be able to explain the properties of matter, different elements, and compounds.
Basically the atoms were investigated in search of particles that explain the behaviour and properties of different chemicals and substances. Existence of atoms was a proposal to explain the real life and scientific observations.
So the atoms and various theories were invented and conceived to explain real life behaviour of elements and compounds. Whatever were proved were retained and further research was pursued assuming that.