If the magnitude of capacitance of capacitor is large potential difference $\mathrm{V}$ is small for a given charge.

$\left[\right.$ Because $\left.\mathrm{C}=\frac{\mathrm{Q}}{\mathrm{V}}\right]$

This means a capacitor with large capacitance can hold large amount of charge $Q$ at a relatively small potential difference.

High potential difference implies strong electric field around the conductors.

A strong electric field can ionise the surrounding air and accelerate the charges so produced to the oppositely charged plates; thereby neutralising the charge on the capacitor plates at least partly.

The charge of the capacitor leaks away due to the reduction in insulating power of the inversing medium and capacitor becomes useless.

The sharp ends of the conductor (here plates) have a large electric charge density. The electric field near such region is very strong. This strong electric field can strip the electrons from the surface of metal this event is called dielectric breakdown and is also called corona discharge.

The maximum electric field upto which an insulating medium can maintain its insulating property is called the dielectric strength.

For air the value of dielectric strength is about $3 \times 10^{6} \mathrm{~V} / \mathrm{m}$ and the electric field corresponds to a potential difference of $3 \times 10^{4} \mathrm{~V}$ between the conductors. Thus, for a capacitors to store a large amount of charge without leaking its capacitance should be high.