The reason to have it is "Illusion of choice"
If 90% of players only go good path and you make a thinly veiled choice of path then these players will think "Wow this game has such good decisions and content behind them" while never knowing about the reality that the other "Path" is basically non-existent.
There's a similar thing for the whole "Eat tadpoles" choice, where there's literally no reason not to just get free powers for free because there's no content behind the choices of eating tadpoles or staying pure besides the albino tadpole stuff.
At least for the "Good vs Evil" thing there's the excuse of "Well alignment wheel doesn't exist in 5e!" as a poor way of justifying lack of Evil actions... But overall it's just insufficient development of choices and content (Which is something that also impacts Act 3 entirely too...)