Need? Obviously not. But they could theoretically be used against purges. Set them out before hand and when a wave is approaching one hit it with an arrow or something. Or you could use them with some other crafting materials to make mines so you don’t need to set them off yourself.

And I suppose if you like the idea of using a trebuchet against purges, you could use them as ammo for that. You know, letting a front line force of thralls hold them off while you us a trebuchet to throw rocks and explosives at the reserves. Something like that.

And as forkyeah mentioned, there’s supposed to be PvE sieges coming in chapter three. What exactly that means is still a bit of an unknown as far as I know, but it should be safe to assume explosives might come in handy.