Why did Mendeleev leave gaps in his early periodic table, and what was the ultimate importance of the gaps?

Answer 1

There were gaps there because the elements had not been discovered as of then. Later on, those gaps were confirmed his prediotions, and the validity of his periodic model.

Answer 2

Mendeleev arranged the elements in columns by increasing atomic mass and in rows by similar chemical properties (opposite to what we do today but the effect is exactly the same). When the next heavier element (higher atomic mass) was to placed in his table, if the element did not have similar chemical properties to the elements in that row, he skipped a row (sometimes two rows) and placed that element in the row with similar properties. Those gaps were for elements that Mendeleev said had not been discovered and he was right. He even predicted the physical and chemical properties of the elements in the gap(s) and when they were discovered those properties were very close to what he had predicted earlier. That really cemented the idea that Mendeleev had something in his version of the periodic table.