For writing expression for total current
For showing that displacement current is the same as the current charging the capacitor
$\mathit{i}={\mathit{i}}_{\mathit{c}}+{\mathit{i}}_{\mathit{d}}$
where, ${\mathit{i}}_{\mathit{c}}$ is conduction current and ${\mathit{i}}_{\mathit{d}}$ is displacement current.
Outside the capacitor ${\mathit{i}}_{\mathit{d}}=0$ so $\mathit{i}={\mathit{i}}_{\mathit{c}}$
Inside the capacitor ${\mathit{i}}_{\mathit{c}}=0$ so $\mathit{i}={\mathit{i}}_{\mathit{d}}$