There's far less incentive to actually pay bribes or hurt specific people if there's no reliable proof of the vote. Even with people taking a photo of a ballot, one can still just strike that ballot and vote again after taking a photo. It's an immense risk that will likely not do you any good, because there's no way to actually know those people voted. The people you're paying and who voted for you would have likely voted for you anyways and you're just otherwise paying people to not bother voting at all or voting against you, while you face immense risk.

If the gangster is just going to hurt a random sampling of people anyways, you might as well just vote however you want to vote. They may or may not commit violence against you regardless of how you vote, its completely disconnected. If you know they can validate it, you're probably going to be less brave.

Just put yourself in those two situations. One where the ballot is absolutely secret, and one where it can be trivially looked up. Someone says you better vote for X or I'll hurt you. You really don't want to vote for X. In the first instance, do you vote for X? In the second, do you still vote for X knowing the thug will be able to know for sure how you voted?

I'm not suggesting nobody would do an illegal thing, obviously I acknowledge people would do illegal things. I'm just pointing to that as why taking a photo of a ballot is illegal in many areas.