Halberd is a kind of specialized multitool. It’s for fighting in formation, looser than pikes, but still fairly close-order. The point allows it to stab and repel riders, the hook for dismounting any riders who feel like lingering too long, and the axehead for the killing blow on infantry or dismounted cavalry.

Think of it as the difference between a Swiss Army Knife and an everyday survival knife. The Swiss Army Knife will do some things better than the survival knife, but the survival knife has more universal versatility, is hardy, and easy to handle.

That being said, the halberd is more useful than the original commenter gives it credit for, the disadvantages compared to a two-handed spear mostly relate to a less-optimal weight and wieldiness (the axe head being rather large and heavy compared to a spearhead) rather than an essential inability to mimic the usage of a spear in most cases. The halberd is more useful against armored targets, or with some room to maneuver in formation. The spear is more useful in extremely tight formations and is more nimble.