What is the significance of leaching in the extraction of aluminium?

In the extraction of aluminium,the significance of leaching is to concentrate pure alumina $(Al_2O_3)$ from bauxite ore.
Bauxite usually contains silica,iron oxide,and titanium oxide as impurities. In the process of leaching,alumina is concentrated by digesting the powdered ore with a concentrated solution of $NaOH$ at $473-523 \, K$ and $35-36 \, bar$. Under these conditions,alumina $(Al_2O_3)$ dissolves as sodium meta-aluminate and silica $(SiO_2)$ dissolves as sodium silicate,leaving the impurities like iron oxide and titanium oxide behind.
$Al_2O_{3(s)} + 2NaOH_{(aq)} + 3H_2O_{(l)} \xrightarrow[35-36 \, bar]{473-523 \, K} 2Na[Al(OH)_4]_{(aq)}$
$SiO_{2(s)} + 2NaOH_{(aq)} \xrightarrow[35-36 \, bar]{473-523 \, K} Na_2SiO_{3(aq)} + H_2O_{(l)}$
The impurities are then filtered and the solution is neutralized by passing $CO_2$ gas. In this process,hydrated $Al_2O_3$ gets precipitated and sodium silicate remains in the solution. Precipitation is induced by seeding the solution with freshly prepared samples of hydrated $Al_2O_3$.
$2Na[Al(OH)_4]_{(aq)} + CO_{2(g)} \to Al_2O_3 \cdot xH_2O_{(s)} + 2NaHCO_{3(aq)}$
Hydrated alumina thus obtained is filtered,dried,and heated to give back pure alumina $(Al_2O_3)$.
$Al_2O_3 \cdot xH_2O_{(s)} \xrightarrow{1470 \, K} Al_2O_{3(s)} + xH_2O_{(g)}$