If you get an RSA receipt, it can be brute forced (in some years), if you don’t you can’t be sure it was counted.

If you get coerced into voting a certain way on paper and taking a photograph*, the coercing person needs to be in the toom to make sure you don’t mark, take a photo, spoil and vote differently. With a receipt you can coerce the receipt, wait some time and buy some computing power to decrypt the vote later.

* secretly as it’s illegal/vote spoiling for obvious reasons