${\mathrm{CaCO}}_3\underset{1073\mathrm{k}}{\overset{∆}{\to }}\mathrm{CaO}+{\mathrm{CO}}_2$

Reverse reaction takes place in a closed vessel . To prevent backward reaction

${\mathrm{CaCO}}_{3\left(\mathrm{s}\right)}\stackrel{1037 \mathrm{K}}{\rightleftharpoons } {\mathrm{CaO}}_{\left(\mathrm{s}\right)}+{\mathrm{CO}}_{2\left(\mathrm{g}\right)}$

To prevent the back ward reaction the CO₂ has to remove from the reaction mixture
According to lechatlier principle the equilibrium shits in the direction to increases the number of moles
When you remove the CO₂ from the reaction mixture, then on the products side number of  gaseous moles decreases, therefore the  equilibrium shifts in forward direction to increase number of moles on product side