The high-explosive shell is the 'regular' kind of mortar shell, that deals 50 damage in a 3.5-tile radius upon exploding.
They are effective against most enemies; siege bases, resting or charging raiders, crashed ship parts and their mechanoid guards. While two mortars are enough against siege bases, around 4 - 8 will be needed for direct attacks on raiders.
They are less effective against enemies equipped with shield belts, as the explosion is blocked by the shield. A high quality shield belt can survive a blast and regenerate faster than the mortar fires. Pairing these with other mortars firing EMP rounds can overcome this problem easily.
It is the standard shell for mortars, dealing high damage and having a decent blast radius. Given the mortar's inaccuracy, it's best to have a battery of mortars firing at once to deal heavy damage to the enemy.
It is quite resource-intensive, so it's best to wait until mid-game until beginning the use of mortars.