It tells you about the reactivity of the element
So in each group (column) on the periodic table, there's a different number of valence electrons. Each atom wants to have 8 valence electrons(or 2 in Hydrogens case). The atoms that can fuse together are reactive towards a
each other. For example,an oxygen atom which has six valence electrons will be reactive towards an element with 2 valence electrons(BUT NOT HELIUM), such as calcium.
Keep in mind that the atoms on the far right side of the periodic table(called noble gases) are not very reactive because they have full electron shells. Helium has a full valence electron shell even though it only has 2 valence elctrons.