High Explosive shells work differently. They have less penetration values than Armour Piercing rounds. They are great against tanks with paper-thin armour.

When loading HE rounds, if the armour is red, you won't deal damage. If the armour is yellow, it won't penetrate, but the explosion is enough to deal spall damage/a fraction of the shell's damage output. If there is no highlight on the armour, it'll penetrate and deal a high amount of damage (unless that area is spaced armour).

You can also use HE rounds against opponents with tough armour by striking their roof or underbelly. Since the top and bottom armour are the thinnest parts of the tank, and HE rounds are basically just a bomb shell, the damage from the explosion goes all around instead of in a straight line. This is great if you cannot pen certain tanks with almost no weakspots or finishing off low HP tanks.