The Periodic Table arranges all the known elements according to their atomic weights. Atomic weights are simply identification tags that tell us how many protons and electrons an atom has. This allows scientists and chemists to predict how certain atoms will behave when paired with others. It also created a systematic way to group atoms having similar characteristics. As aip.org says,
"The structure of the table reflects the particular arrangement of the electrons in each type of atom."
We have to understand that the periodic table is arranged according to how the electrons are structured around the nucleus of the atom. This arrangement of the electrons is what gives each atom its distinct structure and characteristics.
Before Mendeleev developed his table, there were many gaps in the knowledge of the elements and much was misunderstood about chemistry. Although his chart was far from perfect, it helped pave the way for modern chemistry as we know it. Now scientists have a clearer picture about the composition, characteristics, and function of all the known elements on earth.