What's the point of premium ammo for SU-26?

Penetration values for AP shells are for 0-100m. After 100m, the penetration falls off. Penetration values for HEAT (and HE) shells are true any range. At longer ranges (>300m), you are actually getting a higher effective penetration. SPGs like the SU-26 are often firing at these longer ranges.

Note that HEAT and HE shells are not normalized like AP/APCR shells, so HEAT and HE are less effective against angled or sloped armor.

See https://wiki.wargaming.net/en/Battle_Mechanics#Penetration_Mechanics and https://wiki.wargaming.net/en/Ammo for references.

First of all I would like to correct some errors in the previous answers:

Note that HEAT and HE shells are not normalized like AP/APCR shells, so HEAT and HE are less effective against angled or sloped armor.

This is true for HEAT but HE shells can't be bounced thus normalization wouldn't make a difference. Please note that it is possible that you hit with a HE shell for no damage but it does not mean that it is a bounce.

If you check on tanks.gg you will see that there is no difference in soft stats between AP and HEAT shells. The only difference is the HEAT shells have less penetration than the AP shells.

Please note that using HEAT is less effective since the HEAT nerfs and this is especially true for arty because the accuracy of arties is abysmal and you can easily end up hitting the tracks.

You should use HE in most cases when you play arty and only use AP in some special situations where you know that you will probably be able to hit your target.

So to answer your question: there is no point in buying HEAT for your tank.