What's the purpose of poison potions?

What can I do with poison potions? I don't see any point in poison myself. Can I combine it with weapons or use it on enemies?

No, you can't combine the poison with stuff at the moment, but it might be a planned feature.

But you can create a splash potion out of your poison potion. Because you can throw a splash potion it can be used against other entities. Just beware, skeletons and zombies don't take damage from them. To kill these you need a splash health potion. Source

As the wiki says, you can brew poison potions (and other negative effect potions) with gunpowder to create splash potions. You can then throw the splash poison potions at mobs to poison them.

As that wiki page says, undead mobs and spiders are immune to poison splash potions.

You cannot currently use poison with weapons.