Elements are arranged according to certain properties in the periodic table of elements. For example, all the noble gases (elements with fully filled orbitals are arranged in a column). Similarly, all the metals that can lose 1 electron to attain fully filled configuration are arranged together. In the same fashion, properties of elements have a certain trend as we move down a column or move across a row in the periodic table. For example, the diameter increases and electronegativity decreases we move down a column, while the atomic radius decreases as we move across a row.
Thus, the placement of an element in the periodic table gives the scientists an idea of its properties and its reactivity with other elements. Knowing this, we can predict the type of compounds a particular element will form. For example, sodium is placed in group 1 with other metals. It can lose one electron to form a cation and readily forms ionic compounds, such as sodium chloride, etc.
Hope this helps.