There are various reasons for MBT (main battle tanks) to be armed with a variety of different ammunition. The most important reason is to be able to fight different targets with maximised efficiency. HEAT (high explosive anti tank)and APFSDS (armor piercing fin-stabilized discard sabot) are only two possible kinds of ammunition. Other kinds of ammuniton would include HESH (high-explosive squash head, basically the modern version of a case-head, you fire a big chunk of explosive at a target which goes off once it hits), or canister cartriges to be used against infantry and other soft targets (it's the tank equivalent of a shotgun : A fragmenting cartridge filling the air with deadly projectiles) etc etc.

Now there is a drawback for using too many different kinds of ammuniton : Your tank can only load so many rounds. And every specialised round you take with you is one round less of a different kind. If you are in combat and realise you need a lot of the one kind and run out, you are going to have to use the other one.

This is the reason a lot of armies limit the loadout to two different kinds of ammunition : HEAT and APFSDS. APFSDS is soley used against other MBTs and against other highly armored targets such as IFVs (infantry fighting vehicles), which makes sense because they don't really do a lot against non armored targets. Shoot it at a 'soft' target (let's say a truck) it might just go trough doing no damage at all. For this kind of targets you use HEAT, the explosive force deals sufficient damage against all different kinds of targets. Depending on the warhead it might even go through the armor of a MBT, but quite frankly data is sparse here. Even though reactive armor and Chobham composite armor work well against HEAT they are not perfect. They only decrease the thickness needed to stop a HEAT warhead, they do not however render them completly useless.

There are records of MBTs being pierced by HEAT (for example Israeli Merkava's in Lebanon 2006), so we know it's possible. Another big upside for HEAT is that it works regardless of distance. The way HEAT works it does not need kinetic energy from the impact, since the shaped charge gets accelerated to something like Mach 25 anyway). But since accuracy and thus the probability to hit is the bigger problem with modern APFSDS too, this specific upside isn't that important anymore (it was huge in history though. The German Panzerfaust, as well as the American Bazooka in WWII were completly depending on this, since they could not accelerate their warheads fast enough)