what tier you in?
overall below tier 4, u dont't really know the penetration of shell, except some thick armor tank like B1 or Crusader.
after tier 5, thing get start to get in, bounce shot because your shot unable to penetrate enemy tank's armor.
AP have lower damage, but high penetrate value
HE have high damage, but very low penetrate value
we use AP because the penetration value is high, to pen enemy's weak spot (yes, weak spot, even AP unable to penetrate some thick armor of tank) reliably.
for HE, yes, the max damage is high, but only given the shot penetrate, of your shot do not penetrate, the splash damage will cause less damage on enemy, sometime 0 damage. example, sometime i chasing a light tank, i switch to HE, end up the shots do not penetrate, and doing splash damage less than 100
when to use HE? when you shooting some light armor tank, like spg, or light tank, or when you want reset base cap, you do not have much time to aim for weak spot, so use HE to splash damage enemy tank to reset the base capture.
Currently I got a stug III ausf.g .... VK 30.01 H ....... SU - 5 ( getting tier 5 soon SU-122A) ..... and T-28 (getting tier 5 KV-1)
Best tank base on record . Stug III ...... before this was hetzer.