I use my right hand to wipe my ass because my ass is very hairy and I need all the toilet paper precision I can get. I eat with my non-dominant left hand so that I can fuck around on my phone while I eat.
I’ve always seen it as a contract-like promise that you’re telling the truth. So if you weren’t, you can be charged with perjury, which used to come with much more severe punishments.
Much like the odd statements on visa forms where you promise you aren’t a terrorist or have committed genocide.